Highly efficient ethylene/norbornene copolymerization by o-Di(phenyl)phosphanylphenolate-based half-titanocene complexes

Article abstract of DOI:10.1002/pola.26528

A series of o-di(phenyl)phosphanylphenolate-based half-titanocene complexes CpTiCl₂[O-2-R¹-4-R²-6-(Ph₂P)C ₆H₂] (Cp = C₅H₅, 2a: R¹ = R² = H; 2b: R

Full text of DOI:10.1002/pola.26528

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Highly Efficient Ethylene/Norbornene Copolymerization by
o-Di(phenyl)phosphanylphenolate-Based Half-Titanocene Complexes
Xiao-Yan Tang,1,2 Yong-Xia Wang,1 Bai-Xiang Li,1 Jing-Yu Liu,1 Yue-Sheng Li1
1State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of
Sciences, Changchun 130022, China
2Graduate School of the Chinese Academy of Sciences, Changchun Branch, China
Correspondence to: J.-Y. Liu (E-mail: ljy@ciac.jl.cn)
Received 25 September 2012; accepted 10 December 2012; published online 8 January 2013
DOI: 10.1002/pola.26528
ABSTRACT: A series of o-di(phenyl)phosphanylphenolate-based
half-titanocene complexes CpTiCl2[O-2-R1-4-R2-6-(Ph2P)C6H2]
(Cp ¼ C5H5, 2a: R1 ¼ R2 ¼ H; 2b: R1 ¼ F, R2 ¼ H; 2c: R1 ¼ Ph,
plexes exhibited low to moderate activities toward homopoly-
merization of ethylene. Excitingly, they displayed excellent
ability to copolymerize ethylene with norbornene, and catalytic
activity was more than 100 times larger than that of ethylene
homopolymerization in the case of Ph3CB(C6F5)4/iBu3Al as
cocatalyst, affording the copolymers with high comonomer
incorporations. Moreover, DFT calculations study had been
R2 ¼ H; 2d: R1 ¼ SiMe3, R2 ¼ H; 2e: R1 ¼ tBu, R2 ¼ H; 2f: R1
¼
R2
¼
tBu) have been synthesized in high yields (65–87%) by
treating CpTiCl3 with 1.0 equiv of the deprotonated ligands in
THF. The 1H and 31P NMR spectra indicated that the
phosphorus is not coordinated to titanium in complexes 2a–c,
but is coordinated to titanium in complexes 2d–f. Structures
for 2c–f were further confirmed by X-ray crystallography. Com-
plex 2c is essentially a four-coordinate tetrahedral geometry,
whereas complexes 2d-f adopt five-coordinate distorted
square–pyramid geometry around the titanium center. All com-
performed to shed light on the active species and the funda-
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mental role of NBE in improving the catalytic activity.
2013
Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem.
2013, 51, 1585–1594
KEYWORDS: calculations; catalysts; copolymerization
INTRODUCTION Design and synthesis of well-defined, single-
site group IV metal catalysts for olefin polymerization has
attracted considerable attention.1 Among them, half-metallo-
cene-type metal complexes are of particular interest and im-
portance from the industrial point of view. Following the
success of half-sandwich titanium amide (constrained geom-
etry) catalysts (CGC), a great deal of examples of Cp’M(L)X2
have been reported.2–20 Most of them were supported by bi-,
tri-, and tetradentate chelating and anionic ligands contain-
ing, in various combinations, nitrogen and oxygen hard
donors. In contrast, anionic chelating ligands incorporating
L-type soft donors such as phosphine donors have been
much less explored.18,19 Nevertheless, recent work on early
transition-metals such as Group IV metal complexes strongly
suggests that the use of ancillary ligands with softer L
donors, such as phosphorus and sulfur, may offer beneficial
stabilization of the highly reactive metal center.21
polymerization catalysis, we were interested in combining
the high polymerization activity of o-di(phenyl)phosphanyl-
phenolate ligands with cyclopentadienyl group to produce
more active catalysts and more interesting polymerization
behavior. Although [O, P]-type half-titanocene complexes
were first published in 1996,19 their potential application in
polymerization catalysis remains virtually unexplored.
With this in mind, we initiated the studies of synthesizing
and characterizing some titanium complexes for precise, con-
trolled olefin (co)polymerization. Herein, we thus describe
the synthesis and characterization of some novel o-di(phe-
nyl)phosphanylphenolate-based half-titanocene complexes
CpTiCl2[O-2-R1-4-R2-6-(Ph2P)C6H2] (Cp ¼ C5H5, 2a: R1 ¼ R2
¼ H; 2b: R1 ¼ F, R2 ¼ H; 2c: R1 ¼ Ph, R2 ¼ H; 2d: R1
¼
SiMe3, R2 ¼ H; 2e: R1 ¼ tBu, R2 ¼ H; 2f: R1 ¼ R2 ¼ tBu),
and explored their application in ethylene polymerization
and ethylene/NBE copolymerization. These complexes dis-
played excellent ability to copolymerize ethylene with nor-
bornene (NBE), and catalytic activity was more than one
order of magnitude higher than that of ethylene homopoly-
merization, especially in the case of Ph3CB(C6F5)4/iBu3Al as
cocatalyst.
We have become interested in the synthesis of cationic group
IV compounds supported by bidentate ligands with softer
donor atoms to evaluate their potential application in poly-
merization catalysis.22 As part of our investigation of
o-di(phenyl)phosphanylphenolate ligands in homogeneous
Additional Supporting Information may be found in the online version of this article.
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EXPERIMENTAL
Complex 2c was obtained as orange crystals in 72% yield.
1H NMR (300 MHz, CDCl3, 298 K): d 7.61–7.57 (m, 2H, Ar-
H), 7.51–7.34 (m, 14H, Ar-H), 7.07 (t, J ¼ 7.5 Hz, 1H, Ar-H),
6.84 (ddd, J ¼ 7.6, 3.9, 1.6 Hz, 1H, Ar-H), 5.97 (s, 5H, C5H5).
13C NMR (151 MHz, CDCl3, 298 K): d 168.42 (d, J ¼ 21.4
Hz), 137.28, 135.49, 134.13 (d, J ¼ 17.8 Hz, 81H), 133.14,
132.59, 132.00, 130.60, 129.50, 128.86, 128.80, 128.75,
127.64, 124.00, 121.37. 31P NMR (121.5 MHz, CDCl3, 298 K):
d ꢀ10.54 ppm. Anal. Calc. For C29H23Cl2OPTi: C, 64.83; H,
4.32. Found: C, 64.91; H, 4.38.
General Procedures and Materials
All manipulation of air- and/or moisture-sensitive compounds
was carried out under a dry argon atmosphere by using stand-
ard Schlenk techniques or under a dry argon atmosphere in
an MBraun glovebox unless otherwise noted. All solvents were
1
purified from an MBraun SPS system. The H NMR data of the
ligands and complexes used were obtained on a Bruker 300-
MHz spectrometer; the 31P NMR and 13C NMR data of the
complexes were obtained on a Bruker 400ꢀ and 600-MHz
spectrometer, respectively, at ambient temperature, with CDCl3
as the solvent (dried by MS 4 Å). The NMR data of the poly-
mers were obtained on a Varian Unity-400-MHz spectrometer
at 135 ꢁC, with o-C6D4Cl2 as a solvent. Elemental analyses
were recorded on an elemental Vario EL spectrometer. The
weight-average molecular weights (Mw) and the polydispersity
indices of polymer samples were determined at 150 ꢁC by a
PL-GPC 220 type high-temperature chromatograph equipped
with three Plgel 10-lm Mixed-B LS-type columns. 1,2,4-Tri-
chlorobenzene was used as the solvent at a flow rate of 1.0
mL/min. The calibration was made by polystyrene standard
EasiCal PS-1 (PL). CpTiCl3 were purchased from Aldrich. Modi-
fied methylaluminoxane (MMAO, 7% aluminum in heptane so-
lution) was purchased from Akzo Nobel Chemical. Commercial
ethylene was directly used for polymerization without further
purification. The other reagents and solvents were commer-
cially available.
Complex 2d was obtained as orange red crystals in 78%
1
yield. H NMR (300 MHz, CDCl3, 298 K): d 7.76–7.64 (m, 4H,
Ar-H), 7.49 (m, 7H, Ar-H) 7.41–7.34 (m, 1H, Ar-H), 6.98 (t, J
¼ 7.3 Hz, 1H, Ar-H), 6.44 (d, JHP ¼ 2.6 Hz, 5H, C5H5), 0.37
(s, 9H, Si(CH3)3). 13C NMR (151 MHz, CDCl3, 298 K): d
177.23 (d, J ¼ 27.0 Hz), 139.39, 134.18, 132.52 (d, J ¼ 9.5
Hz), 131.12, 129.28 (d, J ¼ 9.5 Hz), 128.61, 123.09, 122.82,
121.83, 121.28, ꢀ0.99. 31P NMR (121.5 MHz, CDCl3, 298 K):
d 25.42 ppm. Anal. Calc. For C26H27Cl2OPSiTi: C, 58.55; H,
5.10. Found: C, 58.41; H, 5.03.
Complex 2e was obtained as red crystals in 87% yield. 1H
NMR (300 MHz, CDCl3, 298 K): d 7.76–7.64 (m, 4H, Ar-H),
7.49 (m, 6H, Ar-H), 7.38 (d, J ¼ 7.7 Hz, 1H, Ar-H), 7.25–7.19
(m, 1H, Ar-H), 6.93 (td, J ¼ 7.6, 1.5 Hz, 1H, Ar-H), 6.44 (d,
t
JHP ¼ 2.6 Hz, 5H, C5H5), 1.45 (s, 9H, Bu-H). 13C NMR (151
MHz, CDCl3, 298 K): d 171.16 (d, J ¼ 27.4 Hz), 138.53 (d, J
¼ 4.2 Hz), 132.51 (d, J ¼ 9.3 Hz), 131.10, 130.94, 130.79,
129.25 (d, J ¼ 9.4 Hz), 125.21, 124.92, 122.18 (d, J ¼ 5.2
Hz), 121.21, 35.26, 29.73. 31P NMR (121.5 MHz, CDCl3, 298
K): d 27.18 ppm. Anal. Calc. For C27H27Cl2OPTi: C, 62.69; H,
5.26. Found: C, 62.58; H, 5.20.
Synthesis of Half-Titanocene Complexes
Synthesis of Compounds 1a–f
Various o-di(phenyl)phosphanylphenol ligands bearing different
substituents on R1 and R2 positions, 2-R1-4-R2-6-(PPh2)C6H2OH
(1a: R1 ¼ R2 ¼ H; 1b: R1 ¼ F, R2 ¼ H; 1c: R1 ¼ Ph, R2 ¼ H;
1d: R1 ¼ SiMe3, R2 ¼ H; 1e: R1 ¼ tBu, R2 ¼ H; 1f: R1 ¼ R2 ¼
tBu) were prepared according to literature procedures.19,22
Complex 2f was obtained as red crystals in 80% yield. 1H
NMR (300 MHz, CDCl3, 298 K): d 7.74–7.64 (m, 4H, Ar-H),
7.49 (m, 6H, Ar-H), 7.40 (d, J ¼ 2.1 Hz, 1H, Ar-H), 7.17 (dd, J
¼ 6.4, 2.2 Hz, 1H, Ar-H), 6.43 (d, JHP ¼ 2.6 Hz, 5H, C5H5),
Synthesis of Half-titanocene Complexes 2a–f
t
t
1.45 (s, 9H, Bu-H), 1.26 (s, 9H, Bu-H). 13C NMR (151 MHz,
CDCl3, 298 K): d 169.29 (d, J ¼ 27.6 Hz), 144.79 (d, J ¼ 4.5
Hz), 137.14 (d, J ¼ 4.5 Hz), 132.54 (d, J ¼ 9.3 Hz), 130.94,
129.17 (d, J ¼ 9.3 Hz), 128.16, 127.33, 124.35, 124.06,
121.11, 35.38, 34.71, 31.61, 29.82. 31P NMR (121.5 MHz,
CDCl3, 298 K): d 26.99 ppm. Anal. Calc. For C31H35Cl2OPTi:
C, 64.94; H, 6.15. Found: C, 64.87; H, 6.10.
Half-titanocene complexes 2a–f were synthesized according
to literature procedures.19 Complex 2a was obtained as or-
ange red crystals in 70% yield. 1H NMR (300 MHz, CDCl3,
298 K): d 7.43–7.28 (m, 11H, Ar-H), 7.04–6.97 (m, 2H, Ar-H),
6.78 (ddd, J ¼ 7.8, 4.0, 1.5 Hz, 1H, Ar-H), 6.50 (s, 5H, C5H5).
13C NMR (151 MHz, CDCl3, 298 K): d 170.03 (d, J ¼ 19.6
Hz), 135.45 (d, J ¼ 7.2 Hz), 134.26 (d, J ¼ 19.4 Hz), 133.22,
130.79, 129.39, 128.80 (d, J ¼ 7.3 Hz), 127.31 (d, J ¼ 9.4
Hz), 124.34, 121.28, 120.17. 31P NMR (121.5 MHz, CDCl3,
298 K): d ꢀ14.91 ppm. Anal. Calc. For C23H19Cl2OPTi: C,
59.90; H, 4.15. Found: C, 59.72; H, 4.08.
Copolymerization of Ethylene with Norbornene
A typical procedure was performed as follows: the prescribed
amounts of toluene, NBE and cocatalyst were added into the
autoclave (100 mL, stainless steel), and the apparatus was
then purged with ethylene. The reaction mixture was then
pressurized to the prescribed ethylene pressure soon after the
addition of a toluene solution containing titanium complex.
The polymerization was terminated with the addition of EtOH,
and the resultant polymer was adequately washed with EtOH
containing HCl and then dried under vacuum for several hours.
The polymerization of ethylene was also performed in the
same manner in the absence of NBE.
Complex 2b was obtained as orange crystals in 65% yield.
1H NMR (300 MHz, CDCl3, 298 K): d 7.44–7.28 (m, 11H, Ar-
H), 7.18–7.09 (m, 1H, Ar-H), 6.90 (td, J ¼ 7.3, 4.8 Hz, 1H, Ar-
H), 6.48 (s, 5H, C5H5). 13C NMR (151 MHz, CDCl3, 298 K): d
157.27 (dd, J ¼ 20.0), 153.90 (d, J ¼ 254.0 Hz), 135.20 (d, J
¼ 9.7 Hz), 134.44 (d, J ¼ 20.2 Hz), 131.47 (d, J ¼ 14.7 Hz),
129.47, 128.80 (d, J ¼ 7.2 Hz), 128.14, 123.89 (d, J ¼ 6.5
Hz), 121.78, 117.08 (d, J ¼ 18.7 Hz). 31P NMR (121.5 MHz,
CDCl3, 298 K):
d
ꢀ17.10 ppm. Anal. Calc. For
C23H18Cl2FOPTi: C, 57.66; H, 3.79. Found: C, 57.42; H, 3.71.
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1
The H NMR spectra of these complexes showed no complex-
ity, and the integration of complexes confirms a 1:1 ratio of
cyclopentadienyl to o-di(phenyl)phosphanylphenolate ligand.
A sharp single resonance at d 5.97–6.50 ppm assigned to the
C5H5(Cp) protons were observed in complexes 2a–c (2a, d
1
6.50 ppm; 2b, d 6.48 ppm; 2c, d 5.97 ppm), whereas the H
NMR spectra of 2d–f display a doublet for the Cp hydrogens
(2d, d 6.44 ppm, JHP ¼ 2.6 Hz; 2e, d 6.44 ppm, JHP ¼ 2.6 Hz;
2f, d 6.43 ppm, JHP ¼ 2.6 Hz) suggesting phosphorus is coor-
dinated to titanium. In addition, the configurations of 2a–f in
solution were also determined by 31P NMR spectroscopy. All
signals in the 31P NMR spectra were shifted substantially
downfield from the values found for the corresponding
ligands. Note that the values of 2d–f (2d, d 25.42 ppm; 2e, d
27.18 ppm; 2f, d 26.99 ppm) are consistent with a triaryl-
phosphine bound to a metal center.19 However, the chemical
shifts of 2a–c (2a, d ꢀ14.91 ppm; 2b, d ꢀ17.10 ppm; 2c, d
ꢀ10.54 ppm) were at higher field than 2d–f, which indicates
that the phosphorus is not coordinated to Ti.
SCHEME 1 Synthesis of complexes 2a–f.
Crystallographic Studies
Single crystals of complexes 2c–f suitable for X-ray structure
determination were grown from the chilled concentrated
mixture of dichloromethane and hexane solution at ꢀ30 ꢁC
in a glove box, thus maintaining a dry, O2-free environment.
The intensity data were collected with the x scan mode
(186 K) on a Bruker Smart APEX diffractometer with CCD
detector using Mo Ka radiation (k ¼ 0.71073 Å). Lorentz,
polarization factors were made for the intensity data and
absorption corrections were performed using SADABS pro-
gram. The crystal structures were solved using the SHELXTL
program and refined using full matrix least squares. The
positions of hydrogen atoms were calculated theoretically
and included in the final cycles of refinement in a riding
model along with attached carbons.
Crystals of 2c–f suitable for crystallographic analysis were
grown from the chilled concentrated CH2Cl2-hexane mixture
solution. The crystallographic data together with the collec-
tion and refinement parameters are summarized in Table 1.
Selected bond distances and angles for 2c–f were summar-
ized in Table 2. If the centroid of the cyclopentadienyl ring is
considered as a single coordination site, complex 2c (Fig. 1)
is the pseudo tetrahedral geometry about the metal atom,
and the phosphorus is not coordinated. However, complexes
2d–f adopt five-coordinate, distorted square–pyramid geome-
try around the titanium center, in which the equatorial posi-
tions are occupied by oxygen and phosphorus atoms of the
ligands and two chlorine atoms. The cyclopentadienyl group
is coordinated on the axial position, as shown in Figures 2
and 3 and Supporting Information Figure S1. The configura-
tions of 2c–f in the solid state were in line with the results
observed in the 31P NMR spectra. These results clearly indi-
cate that the steric bulk in R1 position appears to be
required in generation of five-coordinate, distorted square–
pyramid geometry around the titanium center.
Density Functional Theory Calculations
Density functional theory (DFT) calculations were used for
the mechanism of copolymerization of ethylene and NBE
with complex 2a by using the Amsterdam Density Functional
program package.23 Geometry optimizations and energy cal-
culations were performed using the local density approxima-
tion augmented with Becke’s nonlocal exchange corrections
and Perdew’s nonlocal correction.24,25 A triple STO basis set
was used for Ti, whereas all other atoms were described by
a double-f plus polarization STO basis. The 1s electrons of
the C, P, and O atoms, as well as the 1s-2p electrons of Ti
atom, were treated as frozen core. Finally, first-order scalar
relativistic corrections were added to the total energy of the
system.
Complex 2c closely resembles the structure of other cyclo-
pentadienyl phenoxide complexes with bulky ortho-substitu-
ents rather than the five-coordinate complexes 2d–f. For
example, the Ti(1)AO(1) bond distance in 2c [1.795 (2) Å]
is shorter than those in 2d–f [2d: 1.8708 (13), 2e: 1.872 (3),
2f: 1.8751 (18) Å), but is close to that in CpTiCl2 (2-Ph-
4,6-tBu2C6H2O) [1.785 (2) Å] and Cp*TiCl2(O-2,6-Me2C6H3)
[1.785 (2) Å].26 Ti-Cp (centroid) distances in 2d–f (2d:
2.034, 2e: 2.031, 2f: 2.036 Å) are somewhat longer than that
in 2c (2.016 Å), but shorter than that in CpTiCl2[O-2-tBu-6-
(Ph2P¼¼O)C6H2] (3a, 2.043 Å).17(b) The distance between
Ti(1) and P(1) in 2c is 4.073 Å, suggesting that the phospho-
rus atom is not coordinated to the metal center in the solid
state. The five-membered C2OPTi chelate ring in 2d–f has an
envelope conformation with the metal lying about 0.549–
0.603 Å (2d: 0.549, 2e: 0.603, 2f: 0.593 Å) out of the C2OP
plane in the direction of Cp ring. The Ti(1)AP(1) bond
length in 2d–f is obviously affected by the R1 and R2 groups
RESULTS AND DISCUSSION
Synthesis and Structural Analysis of Complexes
CpTiCl2[O-2-R1-4-R2-6-(Ph2P)C6H2]
Titanium complexes 2a–f have been prepared in high yields
(65–87%) by treating CpTiCl3 with 1.0 equiv of the sodium
salts of o-di(phenyl)phosphanylphenols, which were pre-
pared by the corresponding ligands with NaH, as shown in
Scheme 1.
Pure samples were collected from the chilled concentrated
mixture of dichloromethane and hexane solution placed in
the freezer (ꢀ30 ꢁC). The resultant complexes were identi-
fied by H, 13C, and 31P NMR spectra and elemental analyses.
1
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TABLE 1 Crystal Data and Structure Refinements of Complexes 2c–f
2c
2d
2e
2f
Empirical formula
Formula weight
Crystal system
Space group
C29H23Cl2OPTi
537.24
C27H29Cl4OPSiTi
618.26
C28H29Cl4OPTi
602.18
C32H37Cl4OPTi
658.26
Monoclinic
P2 (1)/n
Triclinic
Triclinic
Triclinic
P-1
P-1
P-1
˚
a (A)
11.2213 (10)
19.6800 (18)
12.0674 (11)
90.00
10.0885 (5)
10.5105 (5)
14.8640 (7)
84.4960 (10)
74.6440 (10)
70.91
10.0569 (7)
10.3311 (7)
16.2854 (11)
106.0040 (10)
92.4440 (10)
118.2480 (10)
1,402.49 (17), 2
1.426
9.9008 (10)
13.4317 (14)
13.8494 (14)
100.981 (2)
103.992 (2)
106.771 (2)
1,642.6 (3), 2
1.331
˚
b (A)
˚
c (A)
a (ꢁ)
b (ꢁ)
c (ꢁ)
109.550 (2)
90.00
3
˚
V (A ), Z
2,511.3 (4), 4
1.421
1,436.18 (12), 2
1.430
Densitycalcd (Mg/m3)
Absorption coefficient (mmꢀ1
F(000)
)
0.638
0.787
0.764
0.658
1,104
636
620
684
Crystal size (mm)
0.26 ꢂ 0.20 ꢂ 0.14
2.07–25.32
12,729
0.32 ꢂ 0.26 ꢂ 0.18
1.42–26.11
8,054
0.36 ꢂ 0.29 ꢂ 0.20
2.30–26.11
7,864
0.36 ꢂ 0.28 ꢂ 0.24
1.96–25.09
8,471
y range for data collection (ꢁ)
Reflections collected
Independent reflections
Data/restraints/parameters
Goodness-of-fit on F2
4,559 (Rint ¼ 0.0602)
4,559/0/307
1.037
5,633 (Rint ¼ 0.0127)
5,633/0/319
1.035
5,475 (Rint ¼ 0.0242)
5,475/0/316
1.058
5,766 (Rint ¼ 0.0229)
5,766/0/352
1.072
Final R indices [I > 2r (I)]: R1, wR2
0.0491, 0.1118
0.387 and ꢀ0.440
0.0316, 0.0811
0.432 and ꢀ0.322
0.0636, 0.1533
1.534 and ꢀ1.325
0.0437, 0.1086
0.594 and ꢀ0.592
3
˚
Largest diff. peak and hole (e/A )
˚
TABLE 2 Selected Bond Distances (A) and Angles (8) for
of the ligand [2d: 2.6446 (6), 2e: 2.6208 (12), 2f: 2.6285 (8)
Å]. The Ti(1)AP(1) bond distances in 2d–f appear in the
range of 2.6208–2.6446 Å, indicative of significant coordina-
tion of phosphorus atom to the metal center in the solid
state. However, the Ti(1)AO(1), O(1)AC(1), and P(1)AC(2)
bond lengths in 2d–f change slightly with the variation in R1
and R2 groups, as shown in Table 2. The two Ti(1)ACl bond
lengths in 2e–f are statistically identical [2e: 2.3435 (12)
and 2.3376 (12) Å, 2f: 2.3297 (8), and 2.3291 (7) Å to Cl(1)
and Cl(2), respectively], whereas in 2c–d the two Ti(1)ACl
bond lengths are noticeably different [2c: 2.2582 (11) and
2.2478 (11) Å, 2d: 2.3320 (6) and 2.3453 (6) Å to Cl(1) and
Cl(2), respectively]. The bond angles for Cl(1)ATi(1)ACl(2)
in 2d–f [2d: 88.96 (2)ꢁ, 2e: 88.84 (5)ꢁ, 2f: 88.70 (3)ꢁ,
respectively] are smaller than that for complex 2c [101.45
(4)ꢁ], but larger than that for complex 3a [84.955 (19)ꢁ].17(b)
Moreover, the bond distances and angles for 2d are consist-
ent with those reported previously.19
Complexes 2c–f
2c
2d
2e
2f
˚
Bond distances (A)
Ti(1)AO(1)
Ti(1)AP(1)
Ti(1)ACl(1)
Ti(1)ACl(2)
1.795 (2) 1.8708 (13) 1.872 (3) 1.8751 (18)
2.6446 (6) 2.6208 (12) 2.6285 (8)
2.2582 (11) 2.3320 (6) 2.3435 (12) 2.3297 (8)
2.2478 (11) 2.3453 (6) 2.3376 (12) 2.3291 (7)
Ti(1)-Cp(centroid) 2.016
2.034
2.031
2.036
O(1)AC(1)
P(1)AC(2)
1.369 (4) 1.360 (2) 1.354 (5) 1.353 (3)
1.835 (3) 1.7994 (19) 1.804 (4) 1.801 (3)
Bond angles (ꢁ)
Cl(1)ATi(1)ACl(2) 101.45 (4) 88.96 (2) 88.84 (5) 88.70 (3)
O(1)ATi(1)AP(1) 72.85 (4) 72.28 (8) 72.45 (5)
Ti (1)AO(1)AC(1) 161.39 (19) 134.99 (11) 135.5 (2) 135.31 (15)
Ti (1)AP(1)AC(2) 97.11 (6) 97.45 (13) 97.24 (9)
Cl(1)ATi(1)AO(1) 104.48 (7) 127.96 (5) 129.66 (9) 129.25 (6)
Cl(2)ATi(1)AO(1) 103.92 (7) 90.17 (4) 89.28 (9) 88.73 (6)
Ethylene (Co)polymerization Catalyzed by 2a–f
Ethylene polymerizations by CpTiCl2[2-R1-4-R2-6-PPh2-
C6H2O] (2a–f) in the presence of MMAO were examined to
explore the effect of the steric environments near the metal
center on the catalytic activity. Complex 2a showed moderate
catalytic activity (2a, 380 kg/molTi h) for ethylene polymer-
Cl(1)ATi(1)AP(1)
Cl(2)ATi(1)AP(1)
78.40 (2) 79.61 (4) 80.38 (3)
144.48 (2) 142.81 (4) 143.53 (3)
O(1)AC(1)AC(2) 117.8 (3) 118.22 (16) 117.3 (3) 117.5 (2)
P(1)AC(2)AC(1) 117.7 (2) 111.70 (14) 111.2 (3) 111.48 (19)
ization (conditions: 5.0 lmol catalyst, ethylene
4 atm,
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FIGURE 1 ORTEP drawings for CpTiCl2[O-2-R1-4-R2-6-(Ph2P)C6H2] [R1 ¼ Ph, R2 ¼ H (2c, left); R1 ¼ SiMe3, R2 ¼ H (2d, middle); R1
¼
R2 ¼ tBu (2f, right)]. Thermal ellipsoids are drawn at the 30% probability level. H atoms and the solvent molecule are omitted for
clarity.
MMAO/Ti ¼ 1000, 20ꢁC, 10min, Vtotal ¼ 50 mL). Introducing
electron-withdrawing group or electron-donating group in R1
position decreased the activity (2b: 220 kg/molTi h, 2c: 290
kg/molTi h, 2d: 160 kg/molTi h, 2e: 200 kg/molTi h, 2f: 130
kg/molTi h, respectively). Complex 2f, bearing tert-butyl
groups in both R1 and R2 position, exhibited lowest activities
among these catalysts, in complete contrast to the results by
CpTiCl2[2-R1-4-R2-6-P(¼O)Ph2-C6H2O], among which com-
plex bearing tert-butyl groups in both R1 and R2 position
exhibited the highest activity.17(b)
(C6F5)4/iBu3Al generated high molecular weight polymer
with high efficiency.17(b) Therefore, herein, we also explored
ethylene/NBE copolymerization using Ph3CB(C6F5)4/i-Bu3Al
as cocatalyst. In the presence of Ph3CB(C6F5)4/iBu3Al, com-
plexes 2a–f only produced trace polymers for ethylene poly-
merization under the similar conditions (conditions: 5.0-
lmol catalyst, ethylene 4 atm, 20 ꢁC, 10min, [Al]/[B]/[Ti] ¼
100/2/1, Vtotal ¼ 50 mL). Surprisingly, notable improve-
ments in the catalytic activity were observed if introducing
NBE into the reaction system, in complete contrast to the
results of the copolymerization by bridged metallocene and
linked half-titanocene reported previously,29,30 in which the
catalytic activity decreased upon increasing the NBE concen-
tration. The copolymerization by 2a/Ph3CB(C6F5)4/iBu3Al
catalyst system also took place, and the observed activity cal-
culated on the basis of the polymer yield was twice higher
than that by 2a/MMAO system (5.0-lmol catalyst, [NBE]initial
Ph3CB(C6F5)4/iBu3Al is also effective cocatalyst for olefin po-
lymerization.27 Fujita et. al. reported that bis[N-(3-tert-butyl-
salicylidene)-anilinato]zirconium(IV) dichloride activated
with Ph3CB(C6F5)4/iBu3Al gave extremely high molecular
weight polyethylene. This complex in combination with
Ph3CB(C6F5)4/iBu3Al could provide a high molecular weight
ethylene-propylene copolymer, Mv 109 ꢂ 104, with 8000 kg
of polymer/mol of catꢃh activity at a propylene content of
20.7 mol %.28 Our recent research indicated that CpZrCl2[O-
2R1-4R2-6(Ph2P¼¼O)C6H2] in combination with Ph3CB
¼
0.5 mol/L, 2a/MMAO: 1660 kg/molTiꢃh; 2a/
Ph3CB(C6F5)4/iBu3Al: 3300 kg/molTiꢃh). Because of better
performance observed by Ph3CB(C6F5)4/iBu3Al as the cocata-
lyst, E/NBE copolymerizations by 2a–f were investigated in
FIGURE 2 13C NMR spectra of E/NBE copolymer with different
NBE incorporations produced by 2d (A: 27.2%, entry 7; B:
51.7%, entry 8; C: 67.8%, entry 16 in Table 3, respectively).
FIGURE 3 Catalytic activities of the complexes 2a–f toward eth-
ylene (co)polymerization at different NBE concentrations.
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TABLE 3 Copolymerization of Ethylene with NBE by 2a–fa
b
Catalyst
Temperature
(ꢁC)
Time
(min)
NBE
Yield
(mg)
Activity
Mw
NBE Incorporation
(%)c
(10ꢀ4
)
Mw/Mn
b
Entries
(lmol)
E (atm)
(mol/L)
(kg/molTi h)
1
2a (1.0)
2a (1.0)
2b (1.0)
2b (1.0)
2c (1.0)
2c (1.0)
2d (1.0)
2d (1.0)
2e (1.0)
2e (1.0)
2f (1.0)
2f (1.0)
2d (1.0)
2d (1.0)
2d (1.0)
2d (5.0)
2d (1.0)
4
4
4
4
4
4
4
4
4
4
4
4
4
4
2
1
4
20
20
20
20
20
20
20
20
20
20
20
20
20
40
20
20
20
10
5
0.5
1.0
0.5
1.0
0.5
1.0
0.5
1.0
0.5
1.0
0.5
1.0
1.0
1.0
1.0
1.0
2.0
350
360
220
300
410
430
480
510
540
520
470
340
460
400
350
150
180
2,100
4,320
1,320
3,600
2,460
5,160
2,880
6,120
3,240
6,240
2,820
4,080
5,520
4,800
2,100
180
22.8
14.6
14.7
11.5
2.4
2.2
2.3
2.1
28.1
53.2
26.4
43.0
2
3
10
5
4
5
10
5
Bimodal
Bimodal
2.3
6
7
10
5
12.6
14.5
16.1
11.1
14.1
12.2
13.8
10.0
9.8
27.2
51.7
22.6
35.8
26.0
44.7
50.4
53.2
56.3
67.8
59.4
8
2.2
9
10
5
2.5
10
11
12
13d
14
15
16
17
a
2.1
10
5
2.3
2.2
5
2.2
5
2.1
10
10
5
2.3
3.8
2.2
2,160
b
16.2
2.1
Conditions: Ph3CB(C6F5)4/iBu3Al as the cocatalyst, [Al]/[B]/[Ti] ¼ 100/2/1, Vtotal ¼ 50 mL; Weight-average molecular weights and polydispersity indi-
ces determined by high temperature GPC at 150 ꢁC in 1,2,4-C6Cl3H3 versus narrow polystyrene standards; NBE content (mol %) estimated by 13C
c
d
NMR spectra; [Al]/[B]/[Ti] ¼ 200/2/1.
detail in the presence of Ph3CB(C6F5)4/iBu3Al. Polyme-
rization conditions such as catalyst concentration and poly-
merization time were varied in order to control the NBE
conversion (< ꢄ10%). The results of the copolymerization
are shown in Table 3 together with the molecular weights
and molecular weight distributions as well as NBE incorpo-
ration of the resultant copolymers.
increasing NBE concentration, whereas the MW values by the
other catalysts decreased on increasing the NBE concentra-
tion. Analog 2e also exhibited remarkable catalytic activity,
but less NBE incorporation than 2d under the same condi-
tions (entries 9–10). The NBE contents in the resultant
copolymers by 2f were similar to those obtained by 2b under
the same conditions (entries 11–12). Therefore, analog 2d
should be the most suited catalyst precursor for the ethylene/
NBE copolymerization in terms of both catalytic activity and
NBE incorporation.
It was revealed that 2a exhibited both high catalytic activity
and efficient NBE incorporation. The observed catalytic activ-
ities and NBE incorporations enhanced upon increasing the
NBE concentration in feed from 0.5 to 1.0 mol/L. The result-
ant copolymers possessed relatively high MWs with unimodal
molecular weight distributions (MWDs), and the MW values
for the copolymers decreased upon increasing NBE contents
(entries 1–2). Both the observed activities and NBE incorpora-
tion by 2b were lower than those by 2a in ethylene/NBE
copolymerization under the same conditions (entries 3–4).
Analog 2c had comparable structures with 2a and 2b, but it
produced bimodal MWDs polymers under the same condi-
tions (entries 5–6). There appears no clear reason at this
moment why 2c afforded poly(E-co-ENB)s with bimodal
MWDs. One possible explanation is that the conjugated phenyl
group in R1 position is much unfavored for stabilizing the
active species. Complex 2d was found to display a very high
activity of 6120 kg/molTi h at high NBE concentration, which
was about 150 times larger than that of ethylene homopoly-
merization. Moreover, 2d showed higher NBE incorporation
than 2b, and the efficiency is at the same level as that by 2a.
The MWs of the resultant copolymers by 2d increased on
We used catalyst 2d to explore the effect of various reaction
parameters like Al/Ti molar ratio, reaction temperature and
ethylene pressure on copolymerization behaviors. As sum-
marized in Table 3, the increase in Al/Ti molar ratio caused
significant change neither in the catalytic activity nor the
MWs for the resultant poly(ethylene-co-NBE)s (entries 13 vs.
8). In addition, NBE incorporation was also independent of
the Al/Ti molar ratio. These results suggest that the domi-
nant chain-transfer pathway is not chain-transfer to alumi-
num alkyls but seems to be b-hydrogen transfer under these
conditions. Both the activity and the MW decreased when
the copolymerization was conducted at the higher tempera-
ture of 40 ꢁC (entry 14). The efficient syntheses of the
copolymers with high NBE contents (50–70 mol %) could be
accomplished upon increasing the NBE concentration or
decreasing low ethylene pressure (entries 8, 15–16).
Although the observed catalytic activities enhanced upon
increasing the NBE concentration, the activity decreased
gradually at higher NBE concentration conditions (entry 17).
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center resulting in the formation the 12-electron, cationic
metal alkyl species (1). In this way, p-type interaction
between the Ti atom and the P atom could help to stabilize
the species. This should be conveniently observed in the 31P
NMR spectra for the changed phosphorus environments. The
31P NMR spectrum of 2e showed a single 31P resonance at
19.5 ppm in the presence of Ph3CB(C6F5)4/iBu3Al. Note that
a resonance at 21.3 ppm also appeared in 31P spectrum of
2a in the presence of Ph3CB(C6F5)4/iBu3Al, apparently indi-
cating the phosphorus is coordinated to titanium in the cata-
lytically active species. Introducing ethylene and NBE into
the catalyst systems afforded the resonances in the 31P NMR
spectra in the range of 16.6–21.3 ppm for 2e and 16.5–25.3
ppm for 2a. Therefore, we assumed that these complexes
possess similar cationic active species, which also provided a
suitable rationale for 2a–f displaying similar behaviors in
ethylene (co)polymerization.
FIGURE 4 Assumed catalytically active species, the optimized
structure of 1.
An extremely low activity was observed (activity < 1 kg/
molTi h) when the polymerization was performed in the ab-
sence of ethylene.
Based on the above results, reaction energies were calcu-
lated using complex CpTiMe[O-6-(Ph2P)C6H4]þ (1) as model.
Reaction energy profiles were obtained by following the
The typical 13C NMR spectra of poly(ethylene-co-NBE)s with
different NBE incorporations are illustrated in Figure 4.29–32
Figure 4(A) shows that the microstructures of the COCs
formed using 2d under low NBE concentration of 0.5 mol/L
possessed few NBE repeat units and contained alternating
ethylene-NBE sequences as well as isolated NBE units. In
contrast, resonances ascribed to NBE diads or triads were
observed for the copolymers prepared at high NBE concen-
tration of 1.0 mol/L and/or under low ethylene pressure
[Fig. 4(B,C)], and the microstructures thus possessed a mix-
ture of NBE repeat units in addition to the alternating, iso-
lated NBE sequences.
ꢀ
well-established Cossee–Arlman or Brookhart–Green mecha-
nism.33 Our discussion has been built around the very first
insertion step by modeling the growing chain end with a
methyl group, corresponding to the initiation step in olefin
polymerization. When the influence of the counterion is
ignored, the insertion of monomer into the TiAC bond of
complex has usually been described as a two-step process:
monomer uptake and insertion.34–37 The uptake involves p-
complexation of monomer to the metal center. The insertion
goes through a transition state with a four-membered-ring
structure.
Theoretical Calculations
Norbornene Competing with Ethylene
Complexes 2a–c and 2d–f possessed different configurations
in the solid state. A question arises: why they could display
similar behaviors in ethylene/NBE copolymerization? More-
over, complexes 2a–f could effectively promote neither ethyl-
ene polymerization nor NBE polymerization in the presence
of Ph3CB(C6F5)4/iBu3Al, why these complexes displayed no-
table activities in ethylene/NBE copolymerization (Fig. 3)?
As shown in Figure 5, the complexation of NBE molecule to
the Ti-alkyl cation lies 5.22 kcal/mol below the free Ti-alkyl
cation and ethylene molecule (channel b), whereas ethylene
complexation adduct is 1.36 kcal/mol below the baseline
(channel a), indicating the complexation of NBE molecule to
the cationic species is much easier. Compared to ethylene,
NBE molecule can strengthen the C¼¼C/Ti binding through
an agostic hydrogen atom on the methylene bridge with the
metal center, which accounts for much of the observed
excess binding energy of NBE on complex 1. The complex 2
not only is 3.86 kcal/mol less stable than the p-complex 5
but also lies in a shallow well, allowing for facile iso-
merization to the p-complex. In addition, the total barrier for
channel a is 3.03 kcal/mol above the energy level of the
reactants, while the barrier is reduced to ꢀ0.04 kcal/mol
when NBE is introduced. Therefore, comparison of the two
reaction channels a and b in Figure 7 shows that channel b
is the preferred r-bond metathesis channel.
Attempt to alkylate above complexes was unsuccessful, some
unidentified products was obtained. As well known, quantum
chemical analysis has become a very powerful tool for pre-
dicting the structures of catalytically active species before
the actual experimental proof. Therefore, we chose four-coor-
dinate complex 2a and five-coordinate complex 2d to
explore their active forms, CpTiMe[O-6-(Ph2P)C6H3]þ and
CpTiMe[O-2-tBu-6-(Ph2P)C6H3]þ, based on DFT calculations.
The conformation with the lowest energy was chosen as the
final model. The computed ground state arrangement of the
CpTiMe[O-2-tBu-6-(Ph2P)C6H3]þ has a pseudotetrahedral ge-
ometry around the Ti center. Unexpectedly, CpTiMe[O-6-
(Ph2P)C6H3]þ also adopted pseudotetrahedral geometry to
Ti, meaning that the phosphorus is coordinated to Ti in the
case of the cationic active species, as shown in Figure 4. It
seems likely, therefore, that two unpaired electrons of phos-
phorus were pushed back to the electron-deficient metal
To corroborate the result that channel b is favored, the rate
constant ratio kN-Me/kE-Me is calculated by the free energy
change of the two monomers. The value of kN-Me/kE-Me is 53,
that is, NBE is more reactive than ethylene in this case. The
formation of terminal adduct complexes 4 and 7 lies roughly
18 and 23 kcal/mol below the baseline, respectively. In
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FIGURE 5 Schematic presentation of polymerization pathway.
addition, a hydrogen bound to the alkyl side chain is able to
provide an additional agostic interaction to the metal in the
transition state.
¼ H; 2d: R1 ¼ SiMe3, R2 ¼ H; 2e: R1 ¼ tBu, R2 ¼ H; 2f: R1
¼ R2 ¼ tBu) have been synthesized in high yields. The 1H
and 31P NMR spectra indicated that the phosphorus is not
coordinated to titanium in complexes 2a–c, but is coordi-
nated to Ti in complexes 2d–f. Additionally molecular struc-
tures show that complex 2c is essentially tetrahedral and
the phosphorus is not coordinated, whereas complexes 2d–f
adopt five-coordinate distorted square–pyramid geometry
around the titanium center. These novel titanium complexes
showed moderate activity for ethylene polymerization,
however they possessed excellent ability to copolymerize
ethylene with NBE, affording the high molecular weight
copolymers with high comonomer incorporations, especially
in the case of Ph3CB(C6F5)4/iBu3Al as cocatalyst.
Chain End Effect
Although the model of the growing chain end with a methyl
group can capture important aspects of the insertion mecha-
nism beyond the very first event of monomer insertion, its
applicability will be limited to a situation where the chain
end consists of an ethylene or similar residue. Therefore, we
show some results when the chain end is replaced by a NBE
residue.
Our calculated energy profiles show that switching the chain
end indeed raises the transition state of NBE well over that
of ethylene (Fig. 6). Apparently, pathway d is preferred for
the second ethylene propagation. The transition states are
1.61 kcal/mol lower in energy than the reactants, suggesting
that ethylene insertion can easily take place in this case.
Moreover, the rate constant ratio kE-N/kE-E is about 3000.
Therefore, it follows from our calculations as discussed
above that NBE as a residue in the growing chain end is pre-
ferred over ethylene as residue for the chain propagation.
Based on DFT calculations, the structures of cationic active
species were predicted. Unexpectedly, these complexes pos-
sessed similar active species with pseudotetrahedral geome-
try around the Ti center. Furthermore, we identified that
introducing NBE into the system not only can reduce the
barrier of the transition state, but also are preferred for the
second ethylene propagation. Thus, it seems that NBE does
an amazing job in improving the catalytic activity, and even a
small amount could accelerate polymerization process. The
present catalyst system is thus a successful example for the
efficient synthesis of random, high molecular weight copoly-
mers with high NBE contents (>50 mol %). We believe that
CONCLUSIONS
A series of o-di(phenyl)phosphanylphenolate-based half-tita-
nocene complexes CpTiCl2[O-2-R1-4-R2-6-(Ph2P)C6H2] (Cp ¼
C5H5, 2a: R1 ¼ R2 ¼ H; 2b: R1 ¼ F, R2 ¼ H; 2c: R1 ¼ Ph, R2
FIGURE 6 Schematic presentation of polymerization pathway.
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R. J. Keaton, K. C. Jayaratne, D. A. Henningsen, L. A. Koterwas,
L. R. Sita, J. Am. Chem. Soc. 2001, 123, 6197–6198.
the results through this study would introduce important in-
formation for designing efficient transition metal catalysts
for the desired (co)polymerization.
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ACKNOWLEDGMENTS
The authors are grateful for subsidy provided by the National
Natural Science Foundation of China (Nos. 21074128 and
20923003). The authors also thank to Kotohiro Nomura (Tokyo
Metropolitan University) for his discussion in preparing the
manuscript.
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