Tritylpyridinium tetrakis(pentafluorophenyl)borate as an efficient activator for "constrained-geometry" catalysts in ethylene polymerization

Article abstract of DOI:10.1016/j.molcata.2003.07.007

Ethylene polymerization with linked amido-cyclopentadienyl or "constrained-geometry" titanium catalysts, Ti(η ⁵:η¹-C₅Me₄SiMe₂N ^tBu)X₂ (X=Me, Bz, Cl), activated by tritylpyridiniu

Full text of DOI:10.1016/j.molcata.2003.07.007

Journal of Molecular Catalysis A: Chemical 208 (2004) 73–81
Tritylpyridinium tetrakis(pentafluorophenyl)borate as an
efficient activator for “constrained-geometry” catalysts
in ethylene polymerization
Kittichote Musikabhumma, Thomas P. Spaniol, Jun Okuda∗
Institut für Anorganische Chemie und Analytische Chemie, Johannes Gutenberg-Universität Mainz, Duesbergweg 10-14, Mainz D-55099, Germany
Received 25 June 2003; accepted 31 July 2003
Abstract
Ethylenepolymerizationwithlinkedamido-cyclopentadienylor“constrained-geometry”titaniumcatalysts, Ti(␩5:␩1-C5Me4SiMe2NtBu)X2
(X = Me, Bz, Cl), activated by tritylpyridinium tetrakis(entafluorophenyl)borate, [Ph3C(NC5H5)] [B(C6F5)4], and silica-supported tritylpyri-
dinium tetrakis(entafluorophenyl)borate (PySTB) was found to proceed with high activity. 1H NMR spectra in CD2Cl2 suggest that ([Ti(␩5:␩1-
C5Me4,SiMe2NtBu)Me (NC5H5)]+ is formed as the main cationic species. Using the heterogeneous cocatalyst PySTB in hexane, unexpectedly
high ethylene polymerization activity was achieved, giving high molecular weight polyethylenes with excellent morphology and high bulk
density (up to 0.46 g/cm3).
© 2003 Elsevier B.V. All rights reserved.
Keywords: Borate activators; Catalysts; Ethylene polymerization; Polyethylene; Supported catalyst
1. Introduction
an effect on the polymerization activity of the titianium
catalysts based on linked amido cyclopentadienyl ligand.
Previously we had noted good ethylene polymerization
activity when Ti(␩5:␩1-C5Me4SiMe2NtBu)Me2 was ac-
tivated in situ with [Ph3C][B(C6F5)4] in the presence of
pyridylethylsilane-modified silica [2b]. We report here
the efficient ethylene polymerization with linked amido-
cyclopentadienyl titanium catalysts activated by tritylpyri-
dinium tetrakis(pentafluorophenyl)borate and by silica-
supported tritylpyridinium tetrakis(pentafluorophenyl)bor-
ate (PySTB).
Recently we have reported a comparative study on homo-
geneous and heterogeneous ethylene polymerization using
linked amido-cyclopentadienyl or “constrained-geometry”
titanium catalysts, Ti(␩5:␩1-C5Me4SiMe2NR)X2 (R = Me,
t
iPr, Bu; X = Me, Bz, Cl) (for reveiws, see [1]), acti-
vated by trityl tetrakis pentafluorophenyl)borate, [Ph3C]
−
≡
[B(C6F5)4], and silica-supported borate, Si–O–B(C6F5)3
Ph3C+ (B-Sylopol). The strong methyl-abstracting trityl
cations in [Ph3C][B(C6F5)4] and Si–O–B(C6F5)3− Ph3C+
≡
resulted in the efficient formation of the cationic alkyl
titanium species [2]. There are several reports on the prop-
erty of tritylpyridinium perchlorate and tetrafluoroborate,
[Ph3C(NC5H5)]A (A = ClO4, BF4), as tritylating agents
which have advantages over trityl chloride, Ph3CCl [3–7].
We wondered whether the combination of the tritylpyri-
dinium cation with the weakly coordinating counter-anion
tetrakis(pentafluorophenyl)borate, B(C6F5)4, used in metal-
locene-catalyzed olefin polymerization [8–10], would have
2. Experimental
2.1. General considerations
All manipulations were performed under argon atmo-
sphere in a glovebox or by standard Schlenk techniques.
Toluene and hexane were purified by distillation under ar-
gon atmosphere over sodium sand. Diethyl ether (Et2O)
was purified by distillation under argon atmosphere over
sodium/benzophenone ketyl. Pyridine was distilled under ar-
gon from CaH2 prior to use. Linked amido-cyclopentadienyl
∗
Corresponding author. Tel.: +49-6131-393-737;
fax: +49-6131-39-25605.
E-mail address: okuda@mail.uni-mainz.de (J. Okuda).
1381-1169/$ – see front matter © 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2003.07.007

74
K. Musikabhumma et al. / Journal of Molecular Catalysis A: Chemical 208 (2004) 73–81
titanium dimethyl, dibenzyl, and dichloro complexes
Ti(␩5:␩1-C5Me4SiMe2NtBu)X2 (X = Me, Bz, Cl) were
synthesized according to established procedures [11–14].
Trityl tetrakis(pentafluorophenyl)borate, [Ph3C] [B(C6F5)4],
triisobutylaluminum (TIBA), 4-{2-(trichlorosilyl)ethyl}-
pyridine, Cl3SiC2H4C5H4N, 25 wt.% in toluene, and
chlorotrimethylsilane were purchased from commerical
sources and used as received. Highly pure grade ethylene
from Linde AG and other chemical reagents were used
without any further purification. Sylopol® 948 silica, do-
nated by GRACE GmbH (300 m2/g surface area, 20 nm
pore diameter, 50 ␮m particle size) was calcined under air
The mixture was then transferred into a tube, pre-cooled
to −30 ◦C and the measurement carried out between −30
and 25 ◦C. 1H NMR (CD2Cl2, 25 ◦C): ␦ 8.73 (br, 2H,
␣-NC5H5+), 8.50 (br, 1H, ␥-NC5H5+), 8.22 (t, JHH
=
3
8.0 Hz, 3H, para-Ph3C+), 8.04 (d, JHH = 4.8 Hz, 2H,
3
3
␣-TiNC5H5), 7.95 (br, 2H, ␤-NC5H5+), 7.75 (t, JHH
=
6.0 Hz, 6H, meta-Ph3C+), 7.65 (d, JHH = 8.4 Hz, 6H,
ortho-Ph3C+), 7.48 (br, ␥-TiNC5H5 and ␥-NC5H5), 7.3-7.1
(m, Ph3C and ␤-NC5H5), 2.46 (s, 3H, Ph3CMe), 2.17, 2.13,
1.82, 1.53 (s, 4 × 3H, C5Me4), 1.45 (s, 9H, CMe3), 1.18 (s,
3H, TiMe), 0.82 (s, 2 × 3H, SiMe2). 19F NMR (CD2Cl2,
25 ◦C): ␦-133.34 (ortho-C6F5), −164.07 (t, 3JFF = 18.8 Hz,
para-C6F5), −167.93 (meta-C6F5), ꢀδ (p-,m-F) = 3.86.
3
atmosphere at 500 ◦C for 5 h and dried under vacuum at
◦
≡
200 C for 6 h prior to use. The silanol content ( Si–OH)
of the calcined silica material was determined by thermo-
gravimetric method [15] and estimated to be 1.18 mmol/g.
2.4. Preparation of pyridylethylsilane-modified silica (PyS)
Pyridylethylsilane-modified silica (PyS) was prepared as
previously described in ref. [2b]. The solid-state 29Si NMR
spectrum showed the signal of pyridylethylsilyl groups
2.2. Tritylpyridinium tetrakis(pentafluorophenyl)borate
[Ph3C(NC5H5)][B(C6F5)4]
≡
Si–C2H4–C5H4N) grafted on silica surface at −54 ppm
The sample for 14N NMR measurement was prepared
by addition of slight excess of pyridine to a solution
of [Ph3C][B(C6F5)4] (300 mg, 0.325 mmol) in 1–2 ml of
toluene-d8. Colorless crystals precipitated from the clear
solution after 30 min. After standing overnight, the colorless
crystals were isolated by filtration, washed with toluene and
dried in vacuo to give [Ph3C(NC5H5)][B(C6F5)4]; yield
215 mg (66%). Anal. Calcd. for C48H20NF20B: C, 57.57;
and that of trimethylsilyl groups ( Si(CH3)3), which were
also introduced on the silica surface, at −15 ppm.
≡
2.5. Preparation of silica-supported tritylpyridinium
borate (PySTB)
To a toluene suspension of 0.9 g content (N content
= 0.576 mmol) of pyridylethylsilane-modified silica, was
added 40 ml toluene solution of [Ph3C][B(C6F5)4] (0.637 g,
0.691 mmol) via syringe at room temperature. The suspen-
sion was then heated to 60 ◦C, and stirred for 6 h. After
cooling down to room temperature, the suspension was
kept stirring for overnight. The change of the color from
dark red to dark yellow-green was observed. After de-
cantation, washing several times with toluene and drying in
vacuo, the silica-supported tritylpyridinium borate (PySTB)
could be obtained as pale yellow-green solid particles.
The boron content was determined by HR-ICP -MS to be
0.16 mmol/g.
1
H, 2.01; N, 1.40. Found: C, 57.06; H, 2.20; N, 1.51. H
NMR (CD2Cl2, −30 ◦C): ␦ 8.71 (d, JHH = 6.0 Hz, 2H,
3
␣-NC5H5+), 8.53 (t, 3JHH = 8.0 Hz, 1H, ␥-NC5H5+), 7.98
3
(t, JHH = 7.2 Hz, 2H, ␤-NC5H5+), 7.5–7.4 (m, 9H, meta-
and para-Ph3C), 7.11 (d, 3JHH = 7.2 Hz, 6H, ortho-Ph3C).
1H NMR (CD2Cl2, 25 ◦C): ␦ 8.72 (br, 2H, ␣-NC5H5+), 8.53
(br, 1H, ␥-NC5H5+), 7.97 (br, 2H, ␤-NC5H5+), 7.49 (br,
1
9H, meta- and para-Ph3C), 7.12 (br, 6H, ortho-Ph3C). H
NMR (CD2Cl2, 40 ◦C): ␦ 8.74 (br, 2H, ␣-NC5H5+), 8.52
(br, 1H, ␥-NC5H5+), 7.99 (br, 2H, ␤-NC5H5+), 7.49 (br,
9H, meta- and para-Ph3C), 7.14 (br, 6H, ortho-Ph3C). 13C
NMR (CD2Cl2, −30 ◦C): ␦ 149.16 (ipso-C, Ph3C), 147.12
(␥-NC5H5+), 144.68 (␣-NC5H5+), 138.06 (␤-NC5H5+),
137.51, 135.06 (C6F5), 130.17 (ortho-Ph3C), 129.47
(meta-Ph3C), 127.93 para-Ph3C), 90.06 (Ph3C). 19F NMR
(CD2C12, −30 ◦C): ␦ −133.77 (ortho-C6F5), −163.39 (t,
2.6. Ethylene polymerization procedures
Ethylene polymerization was carried out in a 100 ml Büchi
glass reactor equipped with a magnetic stirrer. The catalyst
preparation and aging steps were performed in the glovebox.
For the homogeneous polymerization, the required amounts
of solvent, TIBA, toluene stock solution of titanium com-
plex and toluene solution of mixed [Ph3C][B(C6F5)4] and
pyridine were added into the reactor following this order,
and aged at 25 ◦C for 15 min. Then, the reactor was trans-
ferred out of the glovebox, and placed in the oil bath on the
magnetic stirrer to perform the polymerization. In the case
of heterogeneous polymerization, the required amounts of
PySTB, solvent and toluene stock solution of titanium com-
plex were, respectively, added, followed by aging of the cat-
alyst at room temperature (25 ◦C) for 15 min. Then, TIBA
3JFF = 18.8 Hz, para-C6F5), −167.35 (d, JFF = 15.1 Hz,
3
meta-C6F5), ꢀ␦(p-, m-F) = 3.96. 19F NMR (CD2Cl2,
25 ◦C): ␦-133.47 (ortho-C6F5), −163.90 (t, 3JFF = 18.8 Hz,
3
para-C6F5), −167.83 (t, JFF = 18.8 Hz, meta-C6F5), ꢀ␦
(p-,m-F) = 3.93. 14N NMR (CD2Cl2, 25 ◦C): ␦ 228.
2.3. In situ generation of the cationic titanium species
To a cooled solution (−30 ◦C) of [Ph3C (NC5H5)]-
[B(C6F5)4] (73 mg, 0.076 mmol) in 0.5 ml of CD2Cl2,
was added a solution of Ti(␩5:␩1:C5Me4SiMe2NtBu)Me2
(23 mg, 0.069 mmol) in 0.3 ml of CD2Cl2 in the glovebox.

K. Musikabhumma et al. / Journal of Molecular Catalysis A: Chemical 208 (2004) 73–81
75
was added in the last order. Polymerization was started af-
ter introducing ethylene into the evacuated reactor, and the
pressure was controlled by the pressure regulator and kept
constant at 5 bar throughout the polymerization time. Termi-
nation was performed by venting the reactor and adding 1 ml
of methanol. The polymerization mixture was transferred
into a large amount of acidified methanol, and the precip-
itated polyethylene was filtered and washed several times
with methanol, followed by drying at 70 ◦C in air to constant
weight.
nances and are reported relative to tetramethylsilane. 19F and
13C NMR spectra were referenced to CFCl3 and NH3, re-
spectively. NMR measurements of all air-sensitive samples
were carried out in tubes with Teflon-sealed caps. CP-MAS
solid-state 29Si NMR spectra (79.4 MHz) were recorded
on a Bruker ASX-400 spectrometer operating at a field
of 9.4 T. High-resolution-inductively coupled plasma-mass
spectrometry (HR-ICP-MS) was run on an Element 2
(Thermo Finnigan Bremen, Germany) by P. Klemens of this
department. Infrared spectroscopy (IR) was run on a Matt-
son Galaxy 2030 FT-IR spectrometer. Elemental analysis
was performed on a Heraeus Vario EL. Molecular weights
(Mw) and molecular weight distributions (Mw/Mn) of the re-
sulting polyethylenes were determined by high temperature
GPC (PL-GPC210) at 135 ◦C using 1,2,4-trichlorobenzene
as solvent by A. Jekel of the Center for Catalytic Olefin
Polymerization at the Rijksuniversiteit Groningen, the
2.7. Analysis
1H, 13C; 19F and NMR spectra were recorded on a Bruker
DRX-400 spectrometer operating at 400.13, 100.62, 376.45
and 28.91 MHz, respectively. Chemical shifts for 1H and 13
C
NMR spectra were referenced using internal solvent reso-
Fig. 1. 14N NMR spectra of (a) free pyridine, (b) [Ph3C(NC5H5)][B(C6F5)4] formed in situ, (c) isolated [Ph3C(NC5H5)][B(C6F5)4] (in CD2Cl2 at 25 ◦C).

76
K. Musikabhumma et al. / Journal of Molecular Catalysis A: Chemical 208 (2004) 73–81
Scheme 1.
Fig. 2. IR spectra of (a) lithiated silica, (b) pyridylethylsilane-modified silica (PyS), (c) silica-supported tritylpyridinium borate (PySTB).
Netherlands. The crystalline melting temperature (Tm) and
crystallinity (Xc) of the polymer were determined by dif-
ferential scanning calorimetry (DSC, NETZSCH DSC204)
under nitrogen at 10 ◦C/min heating rate. The morphology
of the polymer particles was examined on a LEO 1530
Gemini scanning electron microscope (SEM) by G. Glasser
of the Max-Planck-Institut für Polymerforschung. The poly-
mer bulk density was determined by weighing the polymer
particles in the DSC aluminum pan of known volume.
crystals of the composition [Ph3C(NC5H5)][B(C6F5)4].
The 14N NMR spectra of pyridine and tritylpyridinium salt
[Ph3C(NC5H5)][B(C6F5)4] formed in situ in toluene-d8 and
isolated from the NMR sample are shown in Fig. 1.1 The res-
onance of the nitrogen for free pyridine in CD2Cl2 solution
was observed at 315 ppm (spectrum a). The spectrum b for
[Ph3C (NC5H5)] [B(C6F5)4] shows that beside the resonance
at 315 ppm due to the nitrogen of free pyridine, that of the ni-
trogen in the tritylpyridinium cation observed at higher field
at 228 ppm. This resonance at 228 ppm was also observed in
the spectrum c for the isolated [Ph3C(NC5H5)][B(C6F5)4].
By variable-temperature 1H NMR spectroscopy (between
−30 and +40 ◦C), [Ph3C(NC5H5)][B(C6F5)4] was found
to be stable in CD2Cl2 solution. Neither the dissociation of
3. Results and discussion
3.1. Generation of titanium alkyl cations by
[Ph3C(NC5H5)][B(C6F5)4]
1
Due to extremely low sensitivity of less abundant 15N isotope, highly
concentrated NMR sarnple is required. Therefore, the 14N NMR spec-
troscopy was used in this study.
Addition of pyridine to a solution of the trityl tetrakis(pen-
tafluorophenyl)borate [Ph3C][B(C6F5)4] afforded colorless

K. Musikabhumma et al. / Journal of Molecular Catalysis A: Chemical 208 (2004) 73–81
77
Table 1
Homogeneous ethylene polymerization with Ti(␩5:␩1-C5Me4SiMe2NtBu)Me2/[Ph3C][B(C6F5)4]/TIBA in the presence of pyridinea
Run
number
Solvent
Py/B
Polymer
time (min)
Yield (g)
Activity
(kg PE/mol Ti h)
Mw (×10−3
)
Mw/Mn
Tm (◦C)b
Xc (%)c
1
2
3
4
5
6
7
8
Toluene
Hexane
Toluene
Hexane
Toluene
Toluene
Toluene
Toluene
0
0
1
1
0
0.5
1
30
30
30
30
7
7
7
7
0.495
0.505
0.332
0.160
0.260
0.426
0.329
0.362
198
202
133e
64
446
730
564
621
156
n.d.d
n.d.
n.d.
279
398
209
172
3.1
n.d.
n.d.
n.d.
4.9
4.6
4.0
3.5
138.4
137.8
136.0
136.3
138.1
138.3
137.3
136.3
61.7
78.1
83.1
86.4
69.9
74.7
70.4
64.9
2
a
Polymerization conditions: in 100 ml Büchi glass reactor, total volume 30 ml, ethylene pressure = 5 bar, polymer temperature = 70 ◦C, catalyst aging
time (at 25 ◦C) = 15 min, Ti = 5 ␮mol, B/Ti = 2, Al/Ti = 200.
b
Crystalline melting temperature.
c
Crystallinity determined from Xc(%) = (ꢀHm/ꢀHm∗ ) × 100, ꢀHm∗ = 293 J/g for HDPE.
d
Not determined.
e
The value is significantly lower than the real activity due to the formation of the gel-like product mixture after polymerization time of about 5 min.
tritylpyridinium cation to give free trityl cation and pyridine
nor the formation of the ion–molecule pair was observed
at 40 ◦C, suggesting a rapid equilibrium on the NMR time
scale. The UV-Vis spectrum of the toluene solution of
[Ph3C(NC5H5)][B(C6F5)4] prepared in situ showed the dis-
appearance of the broad absorption band at λmax = 430 nm
attributed to the free trityl cation in solution, in agreement
with the UV spectra of trityl perchlorate, [Ph3C][ClO4],
and trityl tetrafluoroborate, [Ph3C][BF4], in CH2C12 [5].
The cationic titanium spec−ies [Ti(␩5:␩1-C5Me4SiMe2Nt-
Bu)Me(NC5H5)]+[B(C6F5)4 was generated on the NMR
scale by the reaction of Ti(␩5:␩1-C5Me4SiMe2NtBu)Me2
with [Ph3C(NC5H5)][B(C6F5)4] in CD2Cl2. At −30 ◦C,
Ti(␩5:␩1-C5Me4SiMe2NtBu)Me2 did not react with [Ph3C-
(NC5H5)][B(C6F5)4] in CD2Cl2 solution, as the peaks of
␣-, ␥- and ␤-protons of the pyridinium cation were still
observed at δ 8.70, 8.51 and 7.96 ppm. At room tem-
perature, the formation of the cationic titanium species
[Ti(␩5:␩1 - C5Me4SiMe2NtBu)Me(NC5H5)]+ [B(C6F5)4]−
sets in. The singlet for the methyl protons of Ph3CMe
was observed at δ 2.46 ppm, confirming the methyl ab-
straction by the tritylpyridinium cation. The formation of
a pyridine-coordinated cationic methyl titanium species
was confirmed by the singlet at 1.18 ppm. For the aromatic
protons, the signals for the ␣-, ␥- and ␤-protons of the
pyridinium cation became small and broad, while the reso-
nances of ortho-, meta- and para-phenyl protons of the free
trityl cation became more intense [16,17]. These results
indicate the generation of the cationic titanium species of
[Ti(␩5:␩1-C5Me4SiMe2NtBu)Me(NC5H5)]+ (Scheme 1).
3.2. Preparation and characterization of silica-supported
tritylpyridinium borate (PySTB)
The pyridylethylsilane-modified silica, obtained by
modifying calcined silica with trichloropyridylethylsi-
lane, was treated with [Ph3C][B(C6F5)4] in toluene. The
silica-supported tritylpyridinium borate (PySTB) was ob-
tained as pale yellow-green solid particles. The boron
content of the PySTB was determined by HR-ICP-MS
to be 0.16 mmol/g. As seen in the IR spectra in Fig. 2,
the absorption band at about 1640 cm−1 due to the C N
=
stretching of pyridine ring of PySTB (spectrum c) was
found to become remarkably sharper compared to that of
the pyridylethylsilane-modified silica (spectrum b). This
=
observation for the change of the C N stretching absorption
Table 2
Homogeneous ethylene polymerization with Ti(␩5:␩1-C5Me4SiMe2NtBu)X2 (X Me, Bz, Cl)/[Ph3C][B(C6F5)4]/TIBA in the presence and absence of
pyridinea
Run
number
Ti catalyst
Py/B
Polymerization
time (min)
Yield
(g)
Activity (kg
PE/mol Ti h)
Mw (×10−3
)
Mw/Mn
Tm (◦C)b
Xc (%)c
Bulk density
(g/cm3)
1
2
3
4
5
6
X = Me
X = Bz
X = Cl
X = Me
X = Bz
X = Cl
2
2
2
0
0
0
7
7
7
30
30
30
0.362
0.378
0.483
0.495
0.843
0.651
621
648
828
198
337
260
172
155
318
156
145
128
3.5
2.6
9.1d
3.1
3.1
2.2
136.3
136.8
135.4
138.4
136.4
137.2
64.9
62.3
81.1
61.7
60.7
55.2
0.25
0.36
0.26
0.21
0.34
0.29
a
Polymerization conditions: in 100 ml Büchi glass reactor, solvent toluene, total volume = 30 ml, ethylene pressure = 5 bar, polymerization
temperature = 70 ◦C, catalyst aging time (at 25 ◦C) = 15 min, Ti = 5 ␮mol, B/Ti = 2, Al/Ti = 200.
b
Crystalline melting temperature.
c
Crystallinity determined from Xc(%) = (ꢀHm/ꢀHm∗ ) × 100, ꢀHm∗ = 293 J/g for HDPE.
d
Bimodal MWD.

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K. Musikabhumma et al. / Journal of Molecular Catalysis A: Chemical 208 (2004) 73–81
band suggests the formation of positively charged pyridine
nitrogen of the tritylpyridinium borate, [Ph3C(NC5H5)]
[B(C6F5)4] [18].
3.3. Ethylene polymerization with
Ti(η5:η1-C5Me4SiMe2NtBu)Me2 activated by
[Ph3C(NC5H5)][B(C6F5)4]
The results of ethylene polymerization with titanium
dimethyl complex, Ti(␩5:␩1-C5Me4SiMe2NtBu)Me2, ac-
tivated by [Ph3C(NC5H5)][B(C6F5)4], in the presence of
TIBA as impurities scavenger are summarized in Table 1. As
seen from the results in run 1 and 2, the polymerization with-
out pyridine (Py/B = 0), i.e. with only [Ph3C][B(C6F5)4]
as activator, showed almost the same activities in toluene
or hexane, whereas the polymerization at Py/B = 1 was
found to be more efficient in toluene (run 3 and 4). The
activity seemed to be higher than the result from run 1, but
due to gel-formation after polymerization time of 5 min,
the activity of run 3 appeared to be significantly lower. The
effect of the ratio of pyridine to [Ph3C][B(C6F5)4] (Py/B)
on the polymerization activity and the properties of the
resulting polyethylenes are shown in run 5–8. The activity
increased from 446 kg PE/mol Ti·h to 730 kg PE/mol Ti·h
upon the addition of pyridine to [Ph3C][B(C6F5)4] at Py/B
ratio of 0.5. The increase of Py/B ratio from 0.5 to 1 and 2
resulted in a drop of the activity to 564 and 621 kg PE/mol
Ti·h, but still higher than the activity observed from run 5
(Py/B = 0). Varying the ratio of Py/B from 0 to 2 was also
found to affect the molecular weights (Mw) and molecu-
lar weight distributions (MWDs, Mw/Mn) of the resulting
polyethylenes. The Mw increased at Py/B = 0.5, but above
this ratio it continuously decreased to the values lower than
that of run 5 (Py/B = 0). The MWD was found to become
narrower from 4.9 to 3.5 upon increasing the Py/B ratio
Scheme 2.
from 0 to 2. The melting temperatures (Tm) and cystallini-
ties (Xc) of the resulting polyethylenes were found in the
typical range of HDPE.
For comparison, the results of ethylene polymerization
with titanium dimethyl, dibenzyl and dichloro complexes,
Ti(␩5:␩1-C5Me4SiMe2NtBu)X2 (X = Me, Bz, Cl), acti-
vated by [Ph3C(NC5H5)][B(C6F5)4] and [Ph3C][B(C6F5)4]
[2] are summarized in Table 2. It is obvious that remark-
ably higher activities could be obtained in the presence
of pyridine. The dichloro complex was found to give the
highest activity of 828 kg PE/mol Ti·h (run 3), and the ac-
tivity trend follows X = Cl > Bz > Me. In contrast, the
Table 3
Heterogeneous ethylene polymerization with Ti(␩5:␩1-C5Me4SiMe2NtBu)X2 (X = Me, Bz, Cl)/PySTB/TIBA)a
Run
number
Ti catalyst
Solvent
Polymerization
time (min)
Yield (g)
Activity (kg
PE/mol Ti h)
Mw
Mw/Mn
Tm (◦C)b
Xc (%)c
Bulk density
(g/cm3)
(×10−3
)
1
2
3
4
5
6
7f
8f
9f
X = Me
X = Me
X = Bz
X = Bz
X = Cl
X = Cl
X = Me
X = Bz
X = Cl
Toluene
Hexane
Toluene
Hexane
Toluene
Hexane
Toluene
Toluene
Toluene
30
30
30
30
30
30
30
30
30
0.090
2.765
0.124
3.105
0.070
4.413
0.627
1.501
0.213
36
1106
50
1242
28
1765
251
600
85
458
609
896
793
529
429
932
738
647
6.4d
2.8
128.0e
135.9
133.6
133.7
128.5e
137.8
134.8
133.9
133.9
30.3
60.6
41.3
56.5
30.4
69.2
45.3
51.5
42.9
0.27
0.44
0.31
0.46
0.22
0.39
0.29
0.38
0.24
5.7d
3.8
6.6d
3.2
2.3
2.2
3.3
a
Polymerization conditions: in 100 ml Büchi glass reactor, total volume = 30 ml, ethylene pressure = 5 bar, polymerization temperature = 70 ◦C,
catalyst aging time (at 25 ◦C) = 15 min, Ti = 5 ␮mol, B/Ti = 2 (PySTB = 61 mg), Al/Ti = 200.
b
Crystalline melting temperature.
c
Crystallinity determined from Xc(%) = (ꢀHm/ꢀHm∗ ) × 100, ꢀHm∗ = 293 J/g for HDPE.
d
Bimodal MWD.
e
Shoulder at higher melting temperature was observed.
f
Results obtained from B-Sylopol system under the same polymerization conditions [2].

K. Musikabhumma et al. / Journal of Molecular Catalysis A: Chemical 208 (2004) 73–81
79
dibenzyl complex was found to give the highest acitiv-
3.4. Ethylene polymerization with
ity with [Ph3C] [B(C6F5)4] as activator, and the activ-
ity trend follows X = Bz > Cl > Me. Regarding the
properties of the polyethylenes obtained from these two
systems, the Mw, MWD, Tm, Xc and bulk density are ob-
served almost in the same range. The dichloro complex
in the presence of pyridine was found to give polyethy-
lene with unusually broad and bimodal MWD (run 3).
The above results indicate that the tritylpyridinium bo-
rate, [Ph3C(NC5H5)] [B(C6F5)4], is a more efficient ac-
tivator than the trityl borate, [Ph3C][B(C6F5)4], for the
homogeneous ethylene polymerization with titanium cat-
alysts. The higher activities observed might be due to
stabilization of the titanium alkyl cations by coordinated
pyridine, leading to longer life time of the active cationic
species.
Ti(η5:η1-C5Me4SiMe2NtBu)X2 (X = Me, Bz, Cl) activated
by PySTB
The activation process by PySTB to form the silica-
supported cationic titanium catalysts is shown in Scheme 2.
The results of heterogeneous ethylene polymerization
with titanium dimethyl, dibenzyl and dichloro complexes,
Ti(␩5:␩1-C5Me4SiMe2NtBu)X2 (X = Me, Bz, Cl), acti-
vated by PySTB in the presence of TIBA are summarized
in Table 3. Polymerization in toluene was not efficient, as
very low actitivities were observed for all three complexes.
Polymerization in hexane resulted in high activities above
1000 kg PE/mol Ti·h for all three titanium complexes. The
dichloro complex was found to give the highest activity
of 1765 kg PE/mol Ti·h, which is about double of that
Fig. 3. SEM images of polyethylene obtained with Ti(␩5:␩1-C5Me4SiMe2NtBu)Bz2/-PySTB/TIBA in toluene (run 3 in Table 3).

80
K. Musikabhumma et al. / Journal of Molecular Catalysis A: Chemical 208 (2004) 73–81
observed in the corresponding homogeneous system (run 3
in Table 2). The trend in activity for the polymerization in
hexane was observed similar to that of the homogeneous
system, i.e. X = Cl > Bz > Me. For the properties of
the resulting polyethylenes, the molecular weights (Mw) of
the resulting polyethylenes both obtained in toluene and
hexane were found higher than those of the polyethylenes
from homogeneous system, in the range of 4 × 105 to 9 ×
105. Interestingly, the MWDs of the polyethylenes obtained
in toluene were very broad in the range of 5–7 and ob-
served as bimodal MWD curves. Those of the polyethylenes
obtained in hexane were narrow in the typical range for
single-site catalysts. The low activities together with broad
and bimodal MWDs of polyethylenes obtained in toluene
might be explained by the leaching of the cationic active
titanium species from the silica surface. Toluene competes
with the weakly coordinating pyridine on silica surface for
the cationic titanium species. Such a leaching phenomenon,
which can be controlled by the basicity of the functional-
ized amine groups on the solid supports and the polarity
of the polymerization medium, has recently been reported
for ethylene polymerization with cationic hafnocenes sup-
ported on amine-functionalized polystyrene beads [19,20].
The melting temperatures (Tm) and crystallinities (Xc) of the
polyethylenes obtained in toluene are rather low, whereas
those of the polyethylenes obtained in hexane are typical for
HDPE.
With respect to the morphology, the resulting polyethy-
lenes obtained in hexane were found to exhibit good flowa-
bility and high bulk densities (>0.39 g/cm3). The bulk
density as high as 0.46 g/cm3 could be achieved by the
dibenzyl titanium catalyst (run 4). The SEM images of
the resulting polyethylene obtained from run 3 in toluene
(Fig. 3), shows some fine particles and a soft polyethylene
layer, produced in the homogeneous phase due to the leach-
ing, on the surface of the polymer particles. The SEM images
Fig. 4. SEM images of polyethylene obtained with Ti(␩5:␩1-C5Me4SiMe2NtBu)Bz2/PySTB/TIBA in hexane (run 4 in Table 3).

K. Musikabhumma et al. / Journal of Molecular Catalysis A: Chemical 208 (2004) 73–81
81
[2] (a) K. Musikabhumma, T.P. Spaniol, J. Okuda, Macromol. Chem.
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of the polyethylene obtained from run 4 in hexane (Fig. 4)
shows improved morphology and more rigid surface of the
polymer particles.
(b) K. Musikabhumma, T.P. Spaniol, J. Okuda, J. Mol. Cat. 192
(2003) 223.
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4. Conclusions
The ethylene polymerization activity with linked amido-
cyclopentadienyl or “constrained-geometry” titanium cat-
alysts significantly increased in the presence of [Ph3C
(NC5H5)][B(C6F5)4]. The silica-supported tritylpyridinium
borate (PySTB) was found to exhibit higher activity for
ethylene polymerization in hexane when combined with
the corresponding titanium catalysts than the previously
reported B-Sylopol system [2]. This catalyst produced
polyethylenes with excellent morphology and significantly
higher bulk densities. The leaching of the cationic titanium
catalyst was observed in toluene, leading to low activity and
broad and bimodal MWD of the resulting polyethylene.
(b) A.L. McKnight, M.A. Masood, R.M. Waymouth, D.A. Straus,
Organometallics 16 (1997) 2879;
(c) T. Eberle, Doctoral Thesis, University of Mainz, Germany,
1998.
[13] P.-J. Sinnema, Doctoral Thesis, Rijksuniversiteit Groningen, The
Netherlands, 1999.
Acknowledgements
We gratefully acknowledge the Deutsche Forschungs-
gemeinschaft (SFB 625), the International Max-Planck-
Research School on Polymer Science, and the Fonds der
Chemischen Industrie for the financial support. GRACE
GmbH, Worms, has kindly provided us with Sylopol® 948
silica.
[14] J. Okuda, K. Musikabhumma, P.-J. Sinnema, Isr. J. Chem. 42 (2003)
383.
[15] C. du Fresne von Hohenesche, Diploma Thesis, University of Mainz,
Germany, 1999.
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