meso-Me2Si(1-indenyl)2ZrCl2/ methylalumoxane catalyzed polymerization of the ethylene to ethyl-branched polyethylene

Article abstract of DOI:10.1016/j.molcata.2004.12.002

The meso ansa zirconocene with dimethylsilyl bridge, activated by methylalumoxane, catalyses the ethylene polymerization, producing ethyl-branched polyethylene. With respect to the polymers obtained with the previously investigated meso zirconocenes, we have found higher branching amount and lower molecular weight. The rapid β-H transfer from the growing chain to the coordinated monomer could account for both these features of the polymer. The investigation on the structural parameters of the complex, through X-ray diffraction analysis, and on the electrophilicity of the cationic center, through NMR experiments, suggests, as a possible rationalization of this behavior, the obstruction in the inward site.

Full text of DOI:10.1016/j.molcata.2004.12.002

Journal of Molecular Catalysis A: Chemical 230 (2005) 29–33
meso-Me2Si(1-indenyl)2ZrCl2/methylalumoxane catalyzed
polymerization of the ethylene to ethyl-branched polyethylene
Gianluca Melilloa, Lorella Izzoa, Roberto Centoreb,
Angela Tuzib, Alexander Z. Voskoboynikovc, Leone Olivaa,∗
a
Dipartimento di Chimica, Universita` di Salerno, via Allende, I-84081 Baronissi, Italy
b
Dipartimento di Chimica, Universita` di Napoli “Federico II”, via Cinthia, I-80126 Napoli, Italy
c
Department of Chemistry, Moscow State University, V-234, Moscow, GSP, 119899 Russian Federation
Received 8 October 2004; received in revised form 1 December 2004; accepted 1 December 2004
Available online 19 January 2005
Abstract
The meso ansa zirconocene with dimethylsilyl bridge, activated by methylalumoxane, catalyses the ethylene polymerization, producing
ethyl-branched polyethylene. With respect to the polymers obtained with the previously investigated meso zirconocenes, we have found higher
branching amount and lower molecular weight. The rapid ␤-H transfer from the growing chain to the coordinated monomer could account for
both these features of the polymer. The investigation on the structural parameters of the complex, through X-ray diffraction analysis, and on
the electrophilicity of the cationic center, through NMR experiments, suggests, as a possible rationalization of this behavior, the obstruction
in the inward site.
© 2004 Elsevier B.V. All rights reserved.
Keywords: Polyethylene; Polymerization; Metallocenes
1. Introduction
easy path, according to the theoretical studies of Guerra et al.
[12] on propene polymerization. As a consequence should
increase the probability of the H transfer with respect that of
the insertion.
We have recently reported [7] an attempt to organize
the available information on catalyst features into a frame
that could allow rationalizing the different ability of meso
ansa metallocenes to give branched polyethylene. The main
conclusion was that for meso-indenyl group 4 metallocenes
branching was observed only in the case of zirconium deriva-
tives ligated by unsubstituted indenyl ligands.
Among the meso ansa zirconocenes, the investigation on
those connected by the dimethylsilyl bridge has been, until
now, neglected. This short linker was successfully employed
in C2 symmetric precursors for ethylene and propene poly-
merization catalysts [13,14] and it is generally accepted that it
improves the ansa zirconocene performance in some respects,
and in particular in the increase of the molecular weight of
the produced polymers.
The aim of obtaining branched polyethylene by ethylene
homopolymerization is pursued through a variety of catalytic
strategies, by using either late transition metals [1] or group
4 metals with suitable coordination frame [2–10]. A few of
the uncountable ansa metallocenes with diastereotopic sites,
tested as catalyst precursors for ethylene polymerization,
show some capability to produce ethyl-branched polyethy-
lene [5–11]. Such a feature of the polymer chains possibly
arises from the chain growing isomerization through ␤-H
transfer to the incoming monomer followed by insertion of
the unsaturated chain end into the metal–ethyl bond. This H
transfer should occur with meso ansa zirconocene catalyst
because, when the growing chain sits between the two six-
membered rings, the insertion reaction should have not an
∗
Corresponding author. Tel.: +39 089 965237; fax: +39 089 965296.
E-mail address: loliva@unisa.it (L. Oliva).
1381-1169/$ – see front matter © 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2004.12.002

30
G. Melillo et al. / Journal of Molecular Catalysis A: Chemical 230 (2005) 29–33
In the light of this observation we have carried out the
formation having symmetry (non-crystallographic) almost
Cs. Bond lengths between the metal and carbon atoms of
the coordinated pentatomic rings range between 2.481(4)
synthesis, isolation and structural characterization of meso-
dimethylsilylbis(1-indenyl)ZrCl2 and tested its capability to
give, when activated by methylalumoxane, the branched
polyethylene.
˚
and 2.656(4) A and are quite comparable with literature
data on similar complexes [7,18]. This holds also for
the distances between the metal and the baricentre (XA
˚
and XB) of the five-membered rings (Zr–XA = 2.243 A,
2. Results and discussion
˚
Zr–XB = 2.246 A) and also for the bond angle XA–Zr–XB
(126.6◦).
2.1. Synthesis of meso-Me2Si(1-indenyl)2ZrCl2
The dihedral angle between the average planes of the
˚
two indenyl ligands, which are planar within 0.02 A, is
This complex is a byproduct in the preparation of the
racemic mixture. Its synthesis has been performed through
the reaction of ZrCl4 with the tin triethylderivative of the
dimethylsilylbis(1-indenyl) ligand [15–17]. A 7% yield was
obtained of the meso product, purified by crystallization and
characterized by 1H NMR (see Section 4).
64.9(1)◦ and is, again, comparable with literature data
[7] of the ethylene-bridged analogous (62.3◦). A struc-
tural feature of the complex worth of being noticed
is that Si atom is significantly out of the plane of
the two indenyl ligands (Si–C1A–C2A–C3A = −159.3(3)◦,
Si–C1B–C2B–C3B = 159.2(3)◦) without significant distor-
tions of the tetrahedral bonding geometry around Si. This
featureseemsaconsequenceofSiatombeingdirectlybonded
to the two coordinated indenyl ligands: the optimum coordi-
nation geometry of the ligands is maintained, as we have
seen above, at cost of some deformation of bonding ge-
ometry between Si and the indenyl ligands. Actually, this
points to a significant structural difference between this com-
plex and related complexes in which the two indenyl lig-
ands are connected through an ethylene bridge [7,18]. In
fact, in the case of the ethylene bridge, some conforma-
tional freedom for (small) rotations of the indenyl groups
around the M–XA, M–XB directions is present. These ro-
tations are allowed, to some extent, since they only im-
ply variations of the torsion angle around the ethylene
bridge, and this is true, in particular, if the two indenyl
groups rotate in opposite directions; this fact is witnessed
by the crystal structures of complexes of this type [7,18]
in which the conformation found corresponds to a stag-
gered placement of the two indenyl ligands (ca. 10◦) with
respect to each other; furthermore, also in the solid state,
some disorder of the indenyl ligands related to this type of
movement is found [7,18]. In the case of the dimethylsilyl
bridge, on the other hand, no rotation of the indenyl lig-
ands from the eclipsed arrangement is allowed, because it
would require deformation of bond distances and valence
angles around Si atom, and in fact we have not found any
trace of it in the crystal structure analysis (both at low
temperature and at room temperature). So, we can safely
state that complexes with the ethylene bridge have some
conformational flexibility, which is absent in the present
complex.
2.2. Synthesis of meso-Me2Si(1-indenyl)2Zr(CH3)2
The dimethylated complex was obtained through reac-
tion of the meso-Me2Si(1-indenyl)2ZrCl2 with CH3Li in di-
ethylether at low temperature and purified by crystallization
(see Section 4).
2.3. Molecular structure of
meso-Me2Si(1-indenyl)2ZrCl2
The X-ray molecular structure of the complex is shown
in Fig. 1. As it can be seen, the molecule takes a con-
2.4. NMR experiments
Fig. 1. X-ray molecular structure of the complex. Thermal ellipsoids
˚
are drawn at 30% probability level. Selected bond lenghts (A) and an-
gles (◦): Zr–Cl1 2.454(2), Zr-Cl2 2.408(3), Zr–C1A 2.481(4), Zr–C1B
2.483(4), Zr–C2A 2.468(4), Zr–C2B 2.473(4), Zr–C3A 2.562(4), Zr–C3B
2.573(4), Zr–C4A 2.656(4), Zr–C4B 2.651(4), Zr–C5A 2.586(4), Zr–C5B
2.583(4), Si–C10 1.852(5), Si–C11 1.846(5), Si–C1A 1.877(4), Si–C1B
1.880(4), Cl1–Zr–Cl2 94.3(1), C10–Si–C11 11.4(2), C1A–Si–C1B 94.8(2),
Cp–Zr–Cpꢀ 126.2(1)◦.
1H NMR spectrum of meso-dimethylsilylbis(1-indenyl)
zirconium dimethyl recorded in benzene-d6 at room tem-
perature shows for the diastereotopic Zr–Me’s the typi-
cal pattern of a meso structure complex. In the methyl re-
gion there are two quite distinct signals, the upfield peak

G. Melillo et al. / Journal of Molecular Catalysis A: Chemical 230 (2005) 29–33
31
Table 1
2.5. Ethylene polymerisation
Complex
B-CH3 chemical
shift (δ)
Ref.
We have performed a series of ethylene polymeriza-
tions by using the title complex activated by methylalu-
moxane and the results are reported in Table 2. The poly-
merization products have been analyzed through 13C NMR
and calorimetry (DSC). The latter characterization indicates
melting points ranging between 105 and 110 ◦C, typical of
meso-Me2Si(1in−denyl)2Zr(CH3)+
B(C6F5)3CH3
meso-C2H4-
(1-indenyl)2Zr(CH3)+B(C6F5)3CH3
0.74
This work
[7]
0.41
−
branched polyethylene. Actually, upon inspection of the 13
C
NMR spectra one can argue the presence of ethyl branches
(signals at 11.2, 26.6, 27.2, 30.3, 33.8, and 39.5 ppm) and
evaluate, from the areas of the peaks, around 3.5% their
amount, as ratio of the tertiary carbons to the ethylene
units.
(δ −2.18 ppm) can be assigned to the methyl in the in-
ward site, because the shielding resulting from the aromatic
ring current, while the methyl in the outward site flips at δ
0.14 ppm.
For the formation, in a NMR tube, of the species meso-
Me2Si(1-indenyl)2Zr(CH3)+B(C6F5)3CH3−, 1 equivalent of
solid B(C6F5)3 was added to a benzene-d6 solution of 10 mg
The polyethylenes obtained at different polymerization
temperature and, consequently, at different monomer con-
centration show no relevant change in the number of branch-
ing as well as in the melting temperature. This result is in
accordance with what was observed in the presence of meso
Et(Ind)2ZrCl2/MAO. Actually we reported [5] that, with that
catalyst, neither the monomer concentration nor the poly-
merization temperature affects the branches amount. From
a kinetic point of view this fact should indicate that the fre-
quency of ␤-H transfer from the growing chain to the in-
coming monomer parallels the frequency of the monomer
insertion into the metal carbon bond.
In the same table are also reported, for comparison pur-
pose, the features of the polyethylene’s obtained with the pre-
viously described meso-zirconocene based system with simi-
larbiteangle. Thecatalystofthepresentstudyshowsagreater
capability to give branching and the corresponding polymers
have lower molecular weights. This latter feature is a further
consequence of the high frequency of the ␤-H transfer. These
experimental findings are in contrast with the known behavior
of the corresponding C2 symmetric zirconocenes in the ethy-
lene polymerization. Actually the comparison between the
ethylene-bridged and the Me2Si-bridged rac-zirconocenes,
reported by Resconi and coworkers [20], shows the produc-
tion of higher molecular weight polyethylene with the latter,
as an effect of minor ␤-H transfer. On the other hand this same
rac catalyst produces, at very high temperature, polyethylene
1
of the dimethyl complex. The H NMR spectrum (C6D6,
25 ◦C, 300 MHz) of the reaction product shows the follow-
ing resonances: δ 7.30 (d, 2H), δ 6.79 (t, 2H), δ 6.69 (t,
2H), δ 6.55 (m, 2H), δ 6.40 (d, 2H), δ 5.54 (d, 2H), δ 0.74
(broad, BMe−), δ 0.36 (s, SiMe), δ 0.27 (s, SiMe), δ −1.81
(s, ZrMe+).
These NMR evidences suggest the abstraction of the
methyl group in the outward site with formation of the anionic
groupCH3B(C6F5)3− evidencedbythebroadsignalatδ0.74.
Actually the sharp signal at δ −1.81 can be assigned to the
remaining Zr Me and its upfield value suggests the inward
arrangement. According to Brintzinger and coworkers [19]
it is possible to correlate the chemical shift of the methyl on
the borate to the distance between the fragments of the ionic
pair, shorter is this distance higher is the resonance field. The
difference of the chemical shift of the corresponding methyl
in complexes with similar structure can be interpreted as re-
flecting the electrophilicity of the metallic center. In Table 1
such a comparison is reported, by putting together the liter-
ature chemical shift of the ethylene-bridged meso complex,
having similar bite angle (Cp–Zr–Cpꢀ = 126.2◦, as reported
in Ref. [12] versus 126.6◦, see figure). The higher value for
the silyl-bridged complex is indicative of its minor electron-
deficiency.
Table 2
Results of the ethylene polymerizations
Catalysta
T (◦C)
% Branches
Tm (◦C)
Mn
Ref.
meso-Me2Si(1-indenyl)2ZrCl2
−20
0
25
50
−20
0
3.4
3.1
3.8
3.3
1.8
1.6
1.3
1.1
105
109
109
110
121
122
123
121
2650b
This work
This work
This work
This work
1
1
1
6000b
3000b
11700b
42000c
50000c
48000c
49000c
meso Et(1-indenyl)2ZrCl2
25
50
Unpublished results
a
Polymerizations carried out at 1 atm of ethylene pressure.
Determined through NMR, from the relative intensity of the chain end signals and that of the inner carbons.
Determined through GPC.
b
c

32
G. Melillo et al. / Journal of Molecular Catalysis A: Chemical 230 (2005) 29–33
n
with different kind of branches [10] possibly through differ-
ent mechanism.
2.5 M BuLi in hexanes (416 mmol) was added drop wise
for 30 min at vigorous stirring at −40 ◦C. The resulted mix-
ture was stirred for 3 h at ambient temperature, then, cooled
to −40 ◦C, and 69.4 ml (100 g, 416 mmol) of Et3SnCl was
added. This mixture was stirred for 5 h at ambient tempera-
ture and, then, evaporated to dryness. To the residue 100 ml of
toluene was added, and the resulted solution was evaporated
to dryness to remove ether traces. To the residue 750 ml of
toluene was added. Then, the mixture was cooled to −50 ◦C,
and 48.5 g (208 mmol) of ZrCl4 was added. The resulted mix-
ture was stirred for 48 h at room temperature, 3 h at reflux,
and, then, filtered through glass frit (G4). This procedure gave
orange precipitate and red solution. To this filtrate 300 ml of
hexanes was added. Crystals precipitated at −30 ◦C from the
obtained solution were collected, washed by 3 × 30 ml of
hexanes, and dried in vacuum. Yield 6.28 g (7%) of red crys-
tals of pure meso-complex. Anal. calc. for C20H18Cl2SiZr:
C, 53.55; H, 4.04. Found: C, 53.39; H, 3.95. 1H NMR
(CDCl3):δ7.55(m, 2H), 7.53(m, 2H), 7.21(m, 2H), 6.96(dd,
J = 3.3 Hz, J = 0.8 Hz, 2H), 6.93 (m, 2H), 6.13 (d, J = 3.3 Hz),
3. Conclusion
The meso ansa zirconocene with dimethylsylil bridge pro-
duces, when activated by MAO, highly branched polyethy-
lene with low molecular weight. The comparison between the
meso ethylene-bridged and the meso-Me2Si-bridged bis(1-
indenyl) zirconium dichloride, that have similar bite an-
gle, indicates higher ␤-H transfer rate for the silyl-bridged
complex. This result seems in contrast with the behavior
of the C2 symmetric complexes, because the silyl-bridged
rac-zirconocenes produce polymers with higher molecular
weight than the ethylene-bridged ones do.
In principle the greater readiness to the hydrogen trans-
fer could be ascribed either to higher electrofilicity of the
metal or to geometric factors. The NMR study allows to dis-
regard the hypothesis of the higher electrophilicity for the
cation generated from the Si-bridged complex as the cause
of its more frequent H transfer. So possibly, the driving force
for this phenomenon should be the different geometry of the
complexes. Due to the monoatomic bridge, the indenyl moi-
eties are eclipsed, whereas the ethylene bridge allows the
rapid interconversion between two staggered conformations
[18]. As a consequence, with the present catalyst there is
less room in the inward site and the growing chain, when
sits between the two six-members aromatic rings, should
be more strictly compelled into the conformation suitable
to transfer the ␤-hydrogen to the incoming monomer. This
secondary reaction is responsible for the branching forma-
tion as well as of the low molecular weight in the produced
polyethylenes.
1
1.37(s, 3H), 0.97(s, 3H). 13C{ H}NMR(CD2Cl2):δ135.97,
128.89, 128.53, 127.61, 127.25, 127.15, 120.99, 120.83,
92.24, 0.15, −1.80. Next, the orange precipitate was washed
by 2 × 50 ml of dimethoxyethane and dried in vacuum. This
procedure gave 39.7 g (43%) of orange solid of pure rac-
complex. Anal. found: C, 53.67; H, 4.11. 1H NMR (CD2Cl2):
δ 7.54 (m, 2H), 7.50 (m, 2H), 7.32 (m, 2H), 7.05 (m, 2H),
6.85 (dd, J = 3.2 Hz, J= 1.0 Hz, 2H), 6.09 (d, J = 3.2 Hz, 2H),
1.10 (s, 6H).
4.2. Synthesis of meso-Me2Si(1-indenyl)2Zr(CH3)2
An amount of 1 g (2.31 mmol) of meso-Me2Si(1-
indenyl)2ZrCl2 was suspended in 25 mL of diethyl ether. The
reaction mixture was cooled at −78 ◦C, and 2 equiv. of Et2O
solution of CH3Li 1.4 M were added. After 30 min the mix-
ture was slowly warmed to room temperature and stirred for
further 4 h. Then the solvent was removed under reduced
pressure and the residue extracted with 30 mL of anhydrous
toluene. Concentration of the extract followed by cooling to
−20 ◦C gave 0.31 g (yield 50%) of a pure yellow crystalline
product, identified by 1H and 13C NMR.
4. Experimental part
All manipulations involving air-sensitive reagents and
materials were carried out under nitrogen or argon atmo-
sphere using Schlenk or dry box techniques. Solvents were
dried over Na-diphenylketyl (toluene, light petroleum ether,
heptane, THF), over LiAlH4 (diethyl ether), over CaH2
(dichloromethane) and distilled before the use. Deuteriated
solvents were dried by distillation over CaH2 and stored over
1H NMR (C6D6, 25 ◦C, 300 MHz): δ 7.42 (d, 2H), δ 7.14
(d, 2H), δ 6.96 (t, 2H), δ 6.82 (t, 2H), δ 6.70 (m, 2H), δ 5.53
(d, 2H), δ 0.71 (s, SiMeoutward), δ 0.41(s, SiMeinward), δ 0.21
(s, ZrMeoutward), δ −2.15 (s, ZrMeinward).
˚
activated molecular sieves (4 A). MAO was purchased from
Witco as a 10 wt.% solution in toluene, before use the volatile
components were removed in vacuum to yield a powdery
solid. Polymerization grade ethylene was purchased from
SON and further purified by bubbling through a 5 mol% xy-
lene solution of AliBu3.
13C NMR (C6D6, 25 ◦C, 300 MHz): δ 129.8, δ 129.4,
δ 126.2, δ 125.8, δ 125.09, δ 124.8, δ 117.4, δ 112.4, δ
85.1, δ 42.4 (ZrCH3inward), δ 29.1 (ZrCH3outward), δ −1.63
(SiCH3outward), δ −2.39 (SiCH3inward).
4.3. X-ray analysis of meso-Me2Si(1-indenyl)2ZrCl2
4.1. Synthesis of Me2Si(1-indenyl)2ZrCl2
Crystals were obtained by toluene/hexane at −20 ◦C.
Data collection was performed in flowing N2 at −100 ◦C on a
Bruker-Nonius kappaCCD diffractometer (Mo K␣ radiation,
To a solution of 60.0 g (208 mmol) of di(1H-inden-1-
yl)(dimethyl)silane in 1000 cm3 of diethyl ether 166 ml of

G. Melillo et al. / Journal of Molecular Catalysis A: Chemical 230 (2005) 29–33
33
CCD rotation images, thick slices, ␸ scans + ␻ scans to fill the
Acknowledgments
asymmetric unit). Cell parameters from least-squares fit [21]
of θ angles of 83 reflections are in the range 3.51◦≤θ≤19.97◦.
Crystal data: C20H18Cl2SiZr, M = 448.55 g/mol, monoclinic,
P21/c, Z = 4, red crystal, 0.24 mm × 0.20 mm × 0.10 mm,
This work was supported by the Ministero dell’Istruzione,
dell’Universita` e della Ricerca (Prin 2004). Thanks are due
to CIMCF of the Universita` di Napoli “Federico II” for X-ray
equipment.
◦
˚
a = 11.41(1), b = 11.43(1), c = 14.80(2) A, β = 102.5(1) ,
3
V = 1884(3) A , ρcalc = 1.581 g/cm3, µ = 0.929 mm−1. Semi
˚
empirical absorption correction (SADABS) was applied.
12881 reflections collected ( h, k, l), max. θ = 27.51◦,
4298 independent reflections (Rint = 0.0711). Structure
solved by direct method (SIR97 package [22]) and re-
fined by the full matrix least-squares method (SHELXL
program of SHELX97 package [23]) on F2 against all
independent measured reflections. H atoms coordinates
were refined. Two-hundred-and-seventy-three refined
parameters, R = 0.0413 (on reflections with I > 2σ(I))
and R = 0.0745 on all reflections. Max. and min. resid-
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[21] R. Centore, CELLFIT, Universita` degli Studi di Napoli “Federico
II”, Napoli, Italy.
4.5. Polymerizations
Ethylene polymerizations at atmosphere pressure were
carried out in a 100 mL glass flask charged, under nitrogen
atmosphere, with 27 mL of freshly prepared MAO solution
(15.2 mg/mL), prepared by dissolving solid MAO in toluene.
The mixture, magnetically stirred, was thermostated at the
desired temperature, the inert gas was replaced with ethylene
at 1 atm and then were injected 3 mL of a toluene solution
of meso-Me2Si(1-indenyl)2ZrCl2 (1 mg/mL). The pressure
in the flask was kept constant by feeding with the monomer
and after 1 h the reaction mixture was poured into methanol
acidified with HCl. The polyethylene was recovered by fil-
tration and dried under vacuum. Its amount ranges between
0.2 g (polymerization at −20 ◦C) and 1.5 g (at 50 ◦C).
[22] A. Altomare, G. Cascarano, C. Giacovazzo, A. Guagliardi, J. Appl.
Crystallogr. 26 (1993) 343–350.
[23] G.M. Sheldrick, SHELX-97, University of Go¨ttingen, Go¨ttingen,
Germany, 1997.