1H NMR study of the catalytic system (rac-Me2Si(2-Me, 4-PhInd)2ZrCl2- polymethylalumoxane)-triisobutylaluminum

Article abstract of DOI:10.1007/s11172-005-0340-2

The products of the reactions of polymethylalumoxane (MAO) with triisobutylaluminum (TIBA), rac-Me₂Si(2-Me,4-PhInd) ₂ZrCl₂ (1) with MAO (1 + MAO), and (1 + MAO) + TIBA were studied by ¹H NMR at different molar ratios of the components. When the ratio Al_TIBA/Al_MAO is ～6, the reaction between MAO and TIBA involves the replacement of the methyl group of MAO by isobutyl groups and the formation of isobutylmethylalumoxane or mixed isobutylmethylalumoxane structures. When the TIBA content in the system increases to 30 mol.%, these structures are rearranged to form products with a low degree of association. With the equimolar ratio of the reactants, the main reaction products are tetraisobutylalumoxane and polyisobutylalumoxane. The 1 + MAO system with the molar ratio Al_MAO/Zr = 50 affords a MAO-bonded monomethyl monochloride derivative [L₂ZrCl-μ-Me]^δ+[MAO] ^δ-. An increase in this ratio to 150 produces intermediate binuclear complexes [L₂ZrCl-μ-Me-MeZrL₂] ⁺[MAO]^- and [Me₂Al-(μ-Me) ₂-ZrL₂]⁺[MAO]^-. The addition of TIBA induces the replacement of the ZrMe groups by isobutyl groups at the first step of the interaction and formation of nonidentified reaction products at the subsequent steps.

Full text of DOI:10.1007/s11172-005-0340-2

Russian Chemical Bulletin, International Edition, Vol. 54, No. 4, pp. 957—963, April, 2005
957
1H NMR study of the catalytic system (racꢀMe2Si(2ꢀMe,
4ꢀPhInd)2ZrCl2—polymethylalumoxane)—triisobutylaluminum
ꢀ
E. E. Faingol´d, O. M. Chukanova, and N. M. Bravaya
Institute of Problems of Chemical Physics, Russian Academy of Sciences,
5 prosp. Akad. Semenova, 142432 Chernogolovka, Moscow Region, Russian Federation.
Fax: +7 (096) 515 5420. Eꢀmail: nbravaya@cat.icp.ac.ru
The products of the reactions of polymethylalumoxane (MAO) with triisobutylaluminum
(TIBA), racꢀMe2Si(2ꢀMe,4ꢀPhInd)2ZrCl2 (1) with MAO (1 + MAO), and (1 + MAO) + TIBA
were studied by 1H NMR at different molar ratios of the components. When the ratio
AlTIBA/AlMAO is ~6, the reaction between MAO and TIBA involves the replacement of the
methyl group of MAO by isobutyl groups and the formation of isobutylmethylalumoxane or
mixed isobutylmethylalumoxane structures. When the TIBA content in the system increases to
30 mol.%, these structures are rearranged to form products with a low degree of association.
With the equimolar ratio of the reactants, the main reaction products are tetraisobutylalumoxane
and polyisobutylalumoxane. The 1 + MAO system with the molar ratio AlMAO/Zr = 50 affords
a MAOꢀbonded monomethyl monochloride derivative [L2ZrClꢀµꢀMe]δ+[MAO]δ–. An inꢀ
crease in this ratio to 150 produces intermediate binuclear complexes [L2ZrClꢀµꢀMeꢀ
MeZrL2]+[MAO]– and [Me2Alꢀ(µꢀMe)2ꢀZrL2]+[MAO]–. The addition of TIBA induces the
replacement of the ZrMe groups by isobutyl groups at the first step of the interaction and
formation of nonidentified reaction products at the subsequent steps.
Key words: metallocene complexes of Group IVB elements, polymethylalumoxane,
triisobutylaluminum, tetraisobutylalumoxane, polyisobutylalumoxane, isobutylalumoxanes.
Metallocene cationic complexes of Group IVB eleꢀ
ments are highly efficient catalysts for olefin homoꢀ and
copolymerization. A catalyst system is formed by the
reaction of a metallocene complex with an activator
(cocatalyst). The concept that catalytic intermediates
(precursors of the active site) are alkylmetal cationic
complexes bonded to a counterion1—4 was confirmed
by numerous spectroscopic data.5—7 Polymethylalumꢀ
oxane (MAO) ꢀ(ꢀO—Al(Me)ꢀ)nꢀ (n = 4—30) containꢀ
ing a considerable amount of free or partially bound
trimethylaluminum (TMA) (up to 30 mol.% with reꢀ
spect to aluminum) is most frequently used as a coꢀ
catalyst.8 When activating the starting metallocene dichloꢀ
ride L2ZrCl2 (L are haptoꢀbonded cyclopentadienyl,
indenyl, or fluorenyl ligands), MAO bears numerous funcꢀ
tions: it acts as an alkylating agent (Scheme 1, reacꢀ
tion (1)), cationꢀgenerating agent (reaction (2)), and, in
addition, purifies the reaction medium from moisture adꢀ
mixtures.
The MAOꢀactivated catalysts are most active at low
metallocene concentrations (1•10–5—1•10–7 mol L–1
)
and high cocatalyst/catalyst molar ratios (1•103—1•104).
Disadvantages of MAO are its high cost and instability
during storage. Other most popular activators are fluoarylꢀ
boranes and fluoaryl borates.8 However, their use as actiꢀ
vators is restricted, because they have no components
interacting with humidity admixtures in the reaction meꢀ
dium and can activate only dimethylated metallocenes.
Nevertheless, there are examples for the efficient activaꢀ
tion of metallocene dichlorides by a combined activator
including borates and triisobutylaluminum (TIBA).9—13
Some workers14—16 propose an approach to metalloꢀ
cene activation based on the treatment of the metallocene
dichloride complexes with small additives of MAO folꢀ
lowed by activation with TIBA. Thus activated catalysts
are efficient in propylene polymerization and show activꢀ
ity compatible with that of the homogeneous catalytic
systems with MAO. Moreover, a good solubility of TIBA
in aliphatic hydrocarbons enables polymerization to proꢀ
ceed under these conditions. A similar approach using
TIBA is employed for the activation of the supported
metallocene systems. A metallocene complex is deposited
on the support surface with grafted MAO. Thus supported
catalyst is efficiently activated by standard aluminum
Scheme 1
L2ZrCl2 + MAO
L2ZrMeCl
(1)
(2)
L2ZrMeCl + MAO
L2Zr+Me...ClMAO–
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 934—940, April, 2005.
1066ꢀ5285/05/5404ꢀ0957 © 2005 Springer Science+Business Media, Inc.

958
Russ.Chem.Bull., Int.Ed., Vol. 54, No. 4, April, 2005
Faingol´d et al.
trialkyls AlR3 at low cocatalyst/catalyst molar ratios.17—20
However, information on the nature of forming active
sites, reactions on the catalyst surface, and activation
mechanisms is scarce.
The aim of this work is to reveal the activation mechaꢀ
nism of the metallocene complex involving the combined
activator MAO and TIBA and to describe the nature
of intermediates and active sites formed. Zirconocene
racꢀMe2Si(2ꢀMe,4ꢀPhInd)2ZrCl2
(1) was used, which is highly acꢀ
tive and isospecific in propylene
polymerization.
The efficient activation of 1 in
homogeneous propylene polymerꢀ
ization requires very high excess
was distilled off, and the resulting dry residue was evacuated for
8 h and dissolved in tolueneꢀd8. The reaction was carried out at
the molar ratio AlBui3 : H2O = 2 : 1.
Reactions 1 + MAO and (1 + MAO) + TIBA. A weighing
sample of compound 1 was dissolved in toluene in the absence of
oxygen. MAO was added to the resulting metallocene solution at
specified molar ratios AlMAO : Zr. The mixture was stored for
24 h at room temperature. The solvent was distilled off, and the
dry residue was evacuated for 8 h and dissolved in tolueneꢀd8.
TIBA was added at specified AlTIBA : Zr molar ratios to the
zirconocene + MAO mixture 24 h after the reagents were mixed.
Toluene was distilled off in vacuo, and the dry residue was evacuꢀ
ated for 8 h and dissolved in tolueneꢀd8. The zirconocene conꢀ
centration was 0.04 mol L–1
.
1H NMR spectra recording. Samples were placed in NMR
tubes and sealed off from the vacuum line. NMR spectra were
measured in sealed 5ꢀmm cylindrical ampules. The following
conditions were used to record spectra: the spectrum sweep
width 4 kHz, the pulse repetition frequency was 4 s, and the
total number of pulses was 16. Signals of residual protons in
tolueneꢀd8 (δ = 7.06, 7.10, and 7.18) were used to calculate
chemical shifts.
MAO cocatalyst, AlMAO/Zr =
103—104 mol mol–1 21—23
Howꢀ
.
ever, this catalyst can also be actiꢀ
vated by TIBA after preactivation
with MAO at low molar ratios
AlMAO/Zr. To reveal catalytic intermediates and the coꢀ
catalyst nature, we studied the reaction products in the
MAO—TIBA, 1—MAO, and (1—MAO)—TIBA systems
at a varied molar ratio of the components.
Results and Discussion
To elucidate the character of interaction between the
components of the triple catalytic system and to reveal
the nature of the cocatalyst and catalytic intermediates,
Experimental
1
we analyzed the H NMR spectra of the tolueneꢀsoluble
reaction products of the MAO—TIBA, 1—MAO, and
(1—MAO)—TIBA systems. The MAO—TIBA system was
studied at a varied molar ratio AlTIBA/AlMAO. The fragꢀ
ments of the obtained spectra are shown in Fig. 1.
Toluene (special purity grade) was used. The solvent was
distilled over LiAlH4, degassed in a highꢀvacuum line, and stored
above molecular sieves 4A under argon. H NMR spectra were
1
recorded on a Bruker DPXꢀ200 spectrometer (200.13 MHz),
using tolueneꢀd8 (Aldrich) as solvent, which was dehydrated by
multiple distillation over activated molecular sieves 4A. Polyꢀ
methylalumoxane (Witco) was used as a 10% solution in toluꢀ
ene, TIBA (Aldrich) was used as a 1 М solution in toluene, and
racꢀMe2Si(2ꢀMe,4ꢀPhInd)2ZrCl2 (1) (Boulder Scientific Co.)
was prepurified by recrystallization. The 1H NMR spectrum
of TIBA, δ: 0.34 (d, 6 H, CH2, J = 7.1 Hz); 0.82 (d, 18 H, Me,
J = 6.7 Hz); 1.86 (heptet, 3 H, CH). The 1Н NMR specꢀ
trum of compound 1, δ: 0.94 (s, 6 H, Me2Si); 2.09 (s, 6 H,
Fragment 1, a represents the spectrum of MAO, and
fragments 1, b—d are the spectra of the tolueneꢀsoluble
reaction products of MAO and TIBA. The spectrum of
MAO is a superposition of the broad peak (δ –0.16) corꢀ
responding to protons of the Me groups in the oligoꢀ
meric MAO structures ꢀ(ꢀO—Al(Me)ꢀ)nꢀ and a narrow
peak (δ –0.28) of the terminal Me groups in MAO
Me2Alꢀ(ꢀO—Al(Me)ꢀ)nꢀO—AlMe2.19 The spectrum
agrees with the published data.25—28 The MAO sample
was prepared from the starting solution of MAO in toluꢀ
ene by the preꢀevacuation to remove the solvent and
weakly bound TMA, whose content in the starting sample
reaches 30 mol.%. A white powder that formed was disꢀ
solved in tolueneꢀd8. The interaction between MAO and
TIBA results in the transformation of the 1H NMR specꢀ
trum of MAO that involves the disappearance of the former
and appearance of new signals assigned to alumoxane
structures different from MAO.
2ꢀMeInd); 6.89—7.85 (m, 18 H,
C15H9). Compound
racꢀMe2Si(2ꢀMe,4ꢀPhInd)2ZrMe2 (1 (Me)) was synthesized acꢀ
1
cording to a standard procedure.24 The H NMR spectrum of
compound 1 (Me), δ: –0.68 (s, 6 H, Me2Zr); 0.91 (s, 6 H,
Me2Si); 2.02 (s, 6 H, 2ꢀMeInd); 6.91—7.74 (m, 18 H, C15H9).
Argon was dried by passing through a column with activated
molecular sieves (5A). All procedures on the preparation of the
metallocene complexes and catalysts were carried out in the
presence of purified and dried argon.
Reaction MAO + TIBA. A solution of MAO was mixed with
a TIBA solution to give a specified molar ratio AlMAO : AlTIBA
,
the mixture was stored for 1 h at room temperature, toluene
(solvent) was distilled off in vacuo, and the resulting dry residue
was evacuated for 8 h and dissolved in tolueneꢀd8.
Synthesis of tetraisobutylalumoxane (TIBAO). A solution of
TIBA in toluene cooled to –40 °C was added by portions to ice
(H2O), and the reaction mixture was intensely stirred. The reacꢀ
tion product was slowly heated to room temperature, the solvent
1
The H NMR spectrum (see Fig. 1, b) of the soluble
reaction product obtained by the reaction of MAO with
TIBA (AlTIBA : AlMAO ≈ 1 : 15) exhibits the following
changes. The signal of protons of the terminal Me groups
of the oligomeric MAO structures becomes narrower, and
signals of the polyisobutylalumoxane structures appear.

1H NMR studies of (zirconocene+MAO)+TIBA
Russ.Chem.Bull., Int.Ed., Vol. 54, No. 4, April, 2005
959
a
2.2
1.8
1.4
1.0
0.6
0.2
–0.2
–0.6
δ
b
2.2
1.8
1.4
1.0
0.6
0.2
–0.2
–0.6
δ
c
2.28 2.12 1.96
δ
0.45
0.05 –0.35
δ
2.2
1.8
1.4
1.0
0.6
0.2
–0.2
–0.6
δ
d
2.3
2.1
1.9
δ
0.35
–0.05
–0.45
δ
2.4
2.0
1.6
1.2
0.8
0.4
0
–0.4
δ
Fig. 1. Fragments of the 1H NMR spectra of MAO (a) and the reaction systems MAO + TIBA at the varied molar ratio AlTIBA : AlMAO
1 : 15 (b), 1 : 3 (c), and 1 : 1 (d).
=

960
Russ.Chem.Bull., Int.Ed., Vol. 54, No. 4, April, 2005
Faingol´d et al.
In addition, a broad signal of the Me protons of the isobuꢀ
tyl groups of the associates is observed at δ 1.2 along with
the corresponding broad signal of the CH protons at δ 2.1.
The spectrum contains signals of TIBAO 29: a well reꢀ
solved doublet of the Me groups
solved doublet of the Me protons at δ 1.1, a doublet of the
methylenic protons at δ 0.25, and a resolved multiplet of
the isobutyl CH protons at δ 1.97. In addition, according
to the broadening of the signals and their downfield shift,
a small quantity of associated isobutylalumoxane is
present.
at
δ 1.1, a doublet of the
methylenic protons at δ 0.25, and
a multiplet of the CH protons of
the isobutyl groups at 1.97.
The resolved multiplet of the methinic protons indiꢀ
cates the formation of lowꢀmolecular nonassociated strucꢀ
tures similar to higher branched alumoxanes.30
Thus, the main soluble reaction product between MAO
and TIBA (30 mol.%) is tetraisobutylalumoxane. Howꢀ
ever, the associated structures of isobutylalumoxane can
be observed in the 1H NMR spectrum of the soluble prodꢀ
ucts. Evidence of these structures is a superposition of the
broad signals in the region of the methylic and methidic
protons of the isobutyl groups.
TIBAO
Thus, the introduction of even a small amount of TIBA
induces significant changes in the composition and strucꢀ
ture of MAO: the intensive replacement of the Me groups
of MAO by isobutyl groups resulting simultaneously in
the transformation of the polymethylalumosiloxane strucꢀ
tures into polyisobutylalumoxanes or structures of the
mixed type and a decrease in the degree of association of
MAO by the action of TIBA accompanied by the simultaꢀ
neous formation of TIBAO. One can readily imagine that
the disappearance of the signals from the terminal Me
groups of MAO due to the replacement of the Me groups
of MAO by isobutyl groups, the decrease in the size of the
MAO associates, and the appearance of signals of the
tetraisobutylsiloxane structures are interrelated processes
(Scheme 2).
An increase in the molar ratio AlTIBA/AlMAO to 1 reꢀ
sults in the appearance of signals of both tetraisobutylꢀ
1
alumoxane and polyisobutylalumoxane in the H NMR
spectrum (see Fig. 1, d). All the signals of protons in this
isobutylalumoxane are well resolved but downfield shifted:
a multiplet of the CH protons at δ 2.1, a doublet of the Me
protons at δ 1.21, and a group of signals of the methylenic
protons at δ 0.87. They are partially overlap with the
signals of the isobutyl protons of unreacted TIBA. The
positions of the downfield chemical shifts of protons of
the isobutyl groups assume that the Al atom bonded to
these groups is in a more electronegative environment
than in tetraisobutylalumoxane, i.e., the reaction prodꢀ
ucts contain isobutyl groups of the —O—Al(Bui)—O—
fragments. The presence of the resolved multiplet of the
isobutylic CH protons indicates that this compound is not
associated. Since the methylenic protons in these groups
are close to the Al atom, they should manifest a much
greater downfield shift than the methylic and methidic
protons do.
Scheme 2
Thus, with the equimolar ratio of MAO to TIBA, the
main reaction products are isobutylalumoxanes with the
threeꢀcoordinated aluminum atom
Unsaturated coordination of tetraisobutylalumoxane
gives rise to its instability31 and a tendency to form
dimers32 and associated compounds. Our attempt to synꢀ
thesize TIBAO in toluene followed by concentrating its
solution afforded associates (Fig. 2, a). The degree of
association increases with time (Fig. 2, b), which appears
as the disappearance of the resolved multiplet of the
methidic protons and downfield shift of the signals comꢀ
pared to TIBAO, which was synthesized in the reaction of
MAO with TIBA (Fig. 2, c).
The formation of nonassociated isobutylalumoxanes
by the reaction of MAO with TIBA can induce the activaꢀ
tion of MAOꢀpreactivated metallocene dichlorides in the
presence of TIBA. Under conditions typical of metalloꢀ
cene dichloride preactivation by low molar excess MAO,
The disproportionation of MAO under the action of
coordinating trimethylaluminum is known from the litꢀ
erature.25
With an increase in the AlTIBA : AlMAO ratio to 1 : 3
(see Fig. 1, c), the signals of the Me groups of MAO
disappear almost completely. The signals of tetraisobutylꢀ
alumoxane become predominant in the spectrum: a reꢀ

1H NMR studies of (zirconocene+MAO)+TIBA
Russ.Chem.Bull., Int.Ed., Vol. 54, No. 4, April, 2005
961
also decreases with an increase in the AlTIBA/AlMAO ratio,
and the signal shifts to δ 0.17, i.e., to the region where
signals of the bridged Al—Me—Al structures appear.25—28
It is likely that the terminal Me groups are the first to
exchange ligands during the interaction between TIBA
and MAO, and the spectra exhibit the residual bridging
Me groups, while the position of the signal depends on
the degree of association of alumoxane. For the ratio
AlTIBA : AlMAO = 1 : 1 (see Fig. 1, d), a lowꢀintensity
signal appears in the spectrum at δ –0.38. This signal
cannot be ascribed to the terminal AlMe groups, because
the Me groups are completely replaced by isobutyl groups
under these conditions. The appearance of the weak sigꢀ
nal at δ –0.58 observed on synthesizing MAO by the
in situ hydrolysis of AlMe3 has been reported previously.34
Since this signal lies between δ –0.20 (highꢀmolecular
MAO) and δ –0.95 (dimerized tetramethylalumoxane),41
it can be assigned to protons of the bridging Me groups of
MAO with a low molecular weight. By analogy, the weak
signal at δ –0.38 can be ascribed to protons of the bridgꢀ
ing Me groups in the lowꢀmolecularꢀweight MAO, which
is formed by MAO disproportionation with TIBA.
Let us consider the spectra of the reaction products of
1 + MAO and (1 + MAO) + TIBA. To interpret the
spectra and characterize the intermediates, we used the
spectra of the dichloride and dimethyl derivatives of
zirconocene 1. The chemical shifts of the protons of the
bridging groups Me2Si< and ZrMeꢀzirconocenes and the
reaction products of zirconocene dichloride with the
components of the catalytic system are presented in
Tables 1 and 2.
a
2.4
2.4
2.4
2.0
2.0
2.0
1.6
1.6
1.6
1.2
1.2
1.2
0.8
0.8
0.8
0.4
δ
δ
δ
b
0.4
c
0.4
Fig. 2. Fragments of the 1H NMR spectra of TIBAO obtained by
the hydrolysis of TIBA (a, b) and synthesized by the reaction
MAO + TIBA (AlTIBA : AlMAO = 1 : 1) (c). The spectra were
recorded 1 h (a) and 3 days (b) after the synthesis.
the starting dichloride is monomethylated (see Scheme 1,
reaction (1)).5,33—36 In the presence of TIBA, the Me
group is probably replaced by the isobutyl group. Howꢀ
ever, this reaction is insufficient for the subsequent forꢀ
mation of an active site. Monoalkyl monochloride meꢀ
tallocene is activated via the elimination of the Cl– anion
by nonassociated isobutylalumoxanes formed in the reacꢀ
tion of MAO with TIBA. Due to the high content of
threeꢀcoordinated Al atoms,37 nonassociated isobutylꢀ
alumoxanes manifest a higher Lewis acidity than MAO
and eliminate the Cl– anion more easily (see Scheme 1,
reaction (2)).38 It is known29,30,38—40 that branched
alumoxanes can act as efficient cocatalysts for a series of
metallocene complexes, and their activity is close to or
even exceeds the activity of the systems with MAO.
The reaction of compound 1 with MAO was carried
out at the molar ratios AlMAO/Zr = 50 and 150. A comꢀ
parison of the spectra of the starting compound 1 and
its reaction product with MAO showed that the monoꢀ
methylated zirconocene is mainly formed at the ratio
AlMAO/Zr = 50. Its formation is indicated by the appearꢀ
ance of two signals of protons of the Me groups: for
both the bridging Me2Si< group and 2ꢀMe substituent
of the indenyl ligand (see Table 1). In this case, the specꢀ
trum also changes in the region where the indenyl sigꢀ
nals appear. A broadened upfield signal appears simulꢀ
taneously (δ –1.13, see Table 2), which can be asꢀ
signed to protons of the bridging Me groups in the
+
[L2ZrCl(µꢀMe)]δ [MAO]δ– structures (2). The signals of
In addition to the aboveꢀdescribed isobutylalumoꢀ
siloxanes, being the reaction products of MAO with TIBA,
we observed the transformation of the methylalumoxane
structures. The content of these structures decreases with
an increase in the molar ratio AlTIBA/AlMAO but they
appear in the spectra of the reaction products even at an
equimolar ratio of the reactants. With an increase in the
AlTIBA/AlMAO ratio, the signals of the terminal AlMe
groups gradually disappear. The spectra (see Fig. 1, c, d)
contain a signal at δ 0—0.2. The intensity of this signal
Table 1. Chemical shifts (δ) of the Me protons of the 2ꢀMe
substituents in the indenyl ligand and bridging groups >SiMe2
(tolueneꢀd8, δ 7.06)
Catalytic system
δ
2ꢀMe
SiMe2
1
2.09
0.94
1—MAO (AlMAO/Zr = 50)
2.11, 2.19
0.87, 0.95

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Faingol´d et al.
Table 2. Chemical shifts (δ) of the Me protons of the ZrMe
fragments in the reaction products* (tolueneꢀd8, δ 7.06)
tons in different cationic structures. The reactions were
carried out at the ratios AlTIBA/Zr = 10 and 50. When
small quantities of TIBA (AlTIBA/Zr = 10) were introꢀ
duced, the 1H NMR spectrum changed insignificantly
and exhibited the same set of signals of the ZrMe protons
(see Table 2), although their intensity decreased slightly.
An increase in the AlTIBA/Zr ratio to 50 is accompanied
by the disappearance of all the four signals from the Me
protons in the cationic structures along with the earlier
described spectral changes related to the transformations
of polymethylalumoxane. In addition to the lowꢀintensity
signal at δ –0.38, which is detected upon the interaction
between MAO and TIBA, a signal at δ –0.55 appears. All
the signals of the ZrMe fragment disappear, most likely,
due to the replacement of the Me groups by isobutyls in
the intermediates. However, the presence of several comꢀ
ponents in the catalytic system impedes the spectral patꢀ
tern, and it is difficult to determine the presence of signals
of the ZrBui groups in the region of positive chemical
shifts. At the same time, it is known38 that the reaction of
zirconocene dichloride with TIBA affords the isobutyl
derivatives of zirconocene, which further decompose to
form hydrides. A weak signal at δ –0.55 can be attributed
to the bridging hydride protons or bridged structures13
[L2Zrꢀµꢀ(C4H7)AlBui2].
Catalytic system
–δ
Assignment
1 (Me)
0.68
1.13
1.66
1.46
1.13
0.95
1.66
1.46
1.13
0.95
ZrMe
1—MAO (AlMAO/Zr = 50)
1—MAO (AlMAO/Zr = 150)
ZrꢀµꢀMe (2)
ZrꢀµꢀMe (3)
ZrꢀµꢀMe (4)
ZrꢀµꢀMe (2)
ZrMe (3)
ZrꢀµꢀMe (3)
ZrꢀµꢀMe(4)
ZrꢀµꢀMe (2)
ZrMe (3)
1—MAO—TIBA
(AlMAO/Zr = 150, AlTIBA/Zr = 10)
* These signals were not determined for the 1—MAO—TIBA
system (AlMAO/Zr = 150, AlTIBA/Zr = 50).
the nonbridging Me groups in the monomethyl monoꢀ
chloride derivatives are recorded in lower fields.42
An increase in the AlMAO/Zr molar ratio to 150 is
accompanied by the appearance of four upfield signals of
the Me protons (see Table 2). One of them (at δ –1.13) is
attributed to the monomethyl monochloride derivative coꢀ
ordinated to MAO. Three others correspond, most likely,
to different dimeric cationic structures. This assignment is
based on the fact that the signals of the Me protons of the
nondimerized cationic species L2ZrMe+X– appear in lower
fields.43 The signals of the Me protons can be assigned by
analogy to the cationic dimeric structures2 formed by the
reaction of the dimethylzirconocene derivatives and
borates. The positions of the bridging (δ –2.75) and
nonbridging (δ –0.89) Me protons in the dimeric strucꢀ
tures [racꢀMe2SiInd2ZrMe(µꢀMe)MeZrInd2SiMe2]+ in
CD2Cl2 were determined. We assume that the signals obꢀ
served at δ –1.66 and –0.95 are attributed to the bridging
and nonbridging Me groups in similar dimeric structures
formed by the reaction between the cationic center
[MeZrL2]+[MAO]– and monomethyl monochloride deꢀ
rivative [L2ZrCl(µꢀMe)MeZrL2]+[MAO]– (3). The same
intensity of the signals at δ –1.66 and –0.95 is in favor of
the assumed structure. No formation of the dimethyl deꢀ
rivatives was observed under these conditions.36 Thereꢀ
fore, no dimers of the dimethyl complex with cationic
complexes should be formed in this system. Since the
reaction mixture contains TMA, structures of the type
[Me2Al(µꢀMe)ClZrL2]δ+[MAO]δ– (4) similar to those deꢀ
scribed previously2 can be formed, and the signal at δ –1.46
can correspond to the Me protons in a molecule of 4. In
this case, it is difficult to analyze the spectrum in the
regions of signals from the >SiMe2 and 2ꢀMe groups in
the indenyl ligand, because structures of several types give
a complicated spectral pattern.
The observed spectral change enables one to monitor
the influence of the molar ratio of the reactants on the
character of the reaction between TIBA and MAO. At a
low TIBA content, the Me groups are replaced by isoꢀ
butyls, and the terminal groups of MAO are exchanged
especially efficiently. The polymethylalumoxane strucꢀ
ture simultaneously decompose to form tetraisobutylꢀ
alumoxane. The soluble reaction products contain weakly
associated polymethylalumoxane almost without bound
TMA and polyisobutylalumoxane (or mixed Meꢀ,
Buiꢀalumoxane) structures. For the molar ratio
AlTIBA/AlMAO ≈ 0.3—1, the main soluble reaction prodꢀ
ucts are tetraisobutylalumoxane and nonassociated polyꢀ
isobutylalumoxane. Based on the spectra of the reaction
products, lowꢀmolecularꢀweight MAO also appears in
small amounts. The reaction of MAO with compound 1
at the ratio AlMAO/Zr = 50 produces the monomethyl
monochloride zirconocene derivative. The chemical shift
of the signal of ZrMe at δ –1.13 indicates the formation of
+
–
associated bridged structures [L2ZrCl(µꢀMe)]δ [MAO]δ
(2). An increase in the AlMAO/Zr ratio to 150 induces
the formation of two additional types of cationic
complexes: [L2ZrCl(µꢀMe)MeZrL2]+[MAO]– (3) and
[Me2Al(µꢀMe)ClZrL2]+[MAO]– (4). The reaction of
TIBA with the cationic intermediate complexes results in
the disappearance of signals of the Me protons, indicating
that the Me groups were replaced by isobutyl groups at the
first step of the reaction.
The influence of TIBA additives on the state of cataꢀ
lytic sites was studied for the ratio AlMAO/Zr = 150, at
which the spectrum contains the signals of the ZrMe proꢀ
Thus, the activation of the metallocene precursor comꢀ
plexes, under the conditions of MAO preactivation at

1H NMR studies of (zirconocene+MAO)+TIBA
Russ.Chem.Bull., Int.Ed., Vol. 54, No. 4, April, 2005
963
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low molar ratios AlMAO/M followed by activation with
TIBA,14—16 involves higher alkylalumoxanes formed in
the reactions between MAO and TIBA. The main reacꢀ
tion products of dichloride metallocene 1 with MAO
and their transformation by TIBA were determined by
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three—four orders of magnitude lower than those in NMR
experiments. In particular, we can suggest that at low
zirconocene concentrations the probability of formation
of binuclear zirconocene complexes of type 3 is low.
E. E. Faingol´d thanks the Haldor Topse Company
for the financial support of this work.
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Received January 19, 2004;
in revised form March 5, 2005