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Study of Homogeneously Catalyzed ZieglerЦNatta Polymerization of Ethene by ESI-MS.

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Angewandte
Chemie
(X = Cl, Me),[3b] as proved by solid-state X-ray photoelectron
spectroscopy (XPS)[4a] and 13C NMR[4b] studies, as well as by
91
DOI: 10.1002/anie.200503307
Zr and 13C NMR investigations of [Cp2Zr(CH3)2]/MAO
solutions.[5] In the presence of ethene, cation 5 forms the p
Study of Homogeneously Catalyzed Ziegler–
complex 6 and subsequently the insertion product 7 (n = 1),
Natta Polymerization of Ethene by ESI-MS**
which is the first intermediate of the polymerization process.
Step-by-step insertion of ethene then yields the cationic alkyl
zirconocenes 7 (n = 2, 3…n). b Elimination gives the oddLeonardo Silva Santos and Jrgen O. Metzger*
numbered-chain polymer 8, which contains a terminal C C
double bond, and the cationic zirconocene hydride 9, which is
able to initiate polymerization to give, via zirconocene cation
Homogeneous zirconocene catalysts for olefin polymeri10, the even-numbered-chain polymer.[6] However, some
zation are very important. The current standard procedure
for the polymerization of ethene using this catalysts is the
experimental evidence suggests that the general classification
protocol developed by Kaminsky et al,[1] who observed a
of zirconocene/MAO catalyst systems as single-site catalysts
may be an oversimplification.[7]
surprisingly high catalytic activity upon addition of water.
This observation led to the discovery of a highly efficient
Electrospray ionization mass spectrometry (ESI-MS) is
activator, an oligomeric methyl aluminoxane (MAO).[2] This
rapidly becoming an important technique for mechanistic
studies of chemical reactions in solution[8, 9] including homodiscovery rejuvenated Ziegler–Natta catalysis and opened the
era of single-site polymerization catalysis.
geneously catalyzed reactions[10] and high-throughput screenSeveral mechanistic and kinetic studies led to the
ing of homogeneous catalysts.[11] Feichtinger et al.[12] reported
generally accepted reaction mechanism, which is depicted in
on ESI tandem mass-spectrometric investigations (ESI-MS/
Scheme 1. Treatment of a toluene solution of [Cp2ZrCl2] (1)
MS) of Ziegler–Natta-like olefin oligomerization by alkylzirconocene cations generated by the
reaction of [Cp2Zr(CH3)2] with
boron-based activators.[13] Chen studied the polymerization of ethene with
[Cp2ZrCl2]/MAO as the catalyst. The
reaction mixture was quenched with
N,N-dicyclohexylcarbodiimide
(DCC), and the products were investigated by ESI-MS.[11b]
Since ESI normally releases ions
preformed in solution,[14] we expected
that transient ionic species such as 5, 9,
7, and 10 (Scheme 1) should be detectable by ESI-MS in the reacting solution under steady-state conditions,
despite the presence of a large excess
of nonionic species. ESI is also known
for the mild conditions used to form
Scheme 1. Proposed mechanism of the Ziegler–Natta polymerization of C2H4 with the homogethe gaseous ions; ever very labile
neous catalyst [Cp2ZrCl2]/MAO.
molecules can be transferred to the
gas phase.[15]
We report here on a preliminary study of the Ziegler–
with MAO leads to a rapid initial ligand-exchange reaction
Natta polymerization of ethene with the homogeneous
that initially generates the monomethyl complex
catalyst [Cp2ZrCl2]/MAO. We used a microreactor coupled
[Cp2ZrCH3Cl] (2),[3] and an excess of MAO leads to
[Cp2ZrMe2] (4).[3a] Abstraction of chloride from 2 or methyl
directly to the ESI source of a Q-TOF mass spectrometer and
focused on the direct detection and mass-spectrometric
from 4 by MAO gives the catalytically active ion-paired
characterization of the transient cationic and catalytically
species [Cp2ZrCH3]+ (5) with the counterion X-[Al(Me)O-]n
active species involved and on the direct demonstration of
their catalytic activity.
[*] Dr. L. S. Santos, Prof. Dr. J. O. Metzger
First, we set out to investigate the formation of the
Institut f2r Reine und Angewandte Chemie
catalytically active species in a solution that was prepared
Carl von Ossietzky Universit7t Oldenburg
from 1 and MAO in toluene according to a literature
Postfach 2503, 26111 Oldenburg (Germany)
procedure.[16] Direct analysis of the toluene solution containFax: (+ 49) 441-798-3618
ing the catalyst afforded a low-quality spectrum with poor ion
E-mail: juergen.metzger@uni-oldenburg.de
intensity, because toluene is not an ideal solvent for the ESI
[**] Financial support from the German Research Association (DFG) is
process. Dilution of the solution with acetonitrile on-line
gratefully acknowledged (Me 722/18). ESI-MS: Electrospray ionusing a microreactor (Figure 1)[17, 18] resulted in a very clean
ization mass spectrometry.
Reaction Mechanisms
Angew. Chem. Int. Ed. 2006, 45, 977 –981
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
977
Communications
Figure 1. Microreactor coupled on-line to ESI source. In the first
micromixer (left) toluene solutions of the preformed catalyst
[Cp2ZrCl2]/MAO (1:1.2 equiv) and of C2H4 were mixed continuously to
initiate the polymerization. The reaction occurred in the capillary
transferring the reacting solution to the second micromixer (right),
where it is quenched by MeCN and from which it is fed directly to the
ESI source.
spectrum displaying five Zr-containing cationic species that
were easily identified by their isotopic pattern: 5 (m/z 235),
[Cp2ZrCl]+ (m/z 255), 5·MeCN (m/z 276), and
[Cp2ZrCl·MeCN]+ (m/z 296) as well as the ion [Cp2Zr]+ (m/
z 220) (Figure 2).[19]
Figure 2. Positive-mode ESI mass spectrum of a solution of [Cp2ZrCl2]/
MAO (1:1.2 equiv) in toluene after on-line dilution with MeCN.
Ion 5 was characterized by collision-induced dissociation
(CID), which resulted in scission of the Zr CH3 bond, giving
an intense fragment ion of m/z 220[12, 20] (Figure 3 a). The
complexed ions 5·MeCN and [Cp2ZrCl+·MeCN] readily lose
MeCN by CID to give the respective cations 5 and [Cp2ZrCl]+
(Figure 3 b).[21, 22] These CID results give evidence that ions 5
and [Cp2ZrCl]+ found in the ESI mass spectrum (Figure 2)
may arise by in-source decay of the respective MeCN adduct.
[Cp2Zr]+ may be formed analogously by fragmentation of ion
5.
The formation of ion 5 in solution is a fast reaction. Thus,
when solutions of 1 and of MAO, both in toluene, were mixed
in a microreactor and the reaction was quenched after 1.7 s
with acetonitrile in a second microreactor (Figure 1), a mass
spectrum similar to that in Figure 2 was obtained.
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Figure 3. Positive-mode ESI tandem mass spectra of zirconocene
intermediate cations: a) 5 of m/z 235, b) 5·MeCN of m/z 276, c) 7
(n = 1) of m/z 263, and d) 7 (n = 20) of m/z 796 (exact mass
795.6324).
However, to probe directly that ion 5 is indeed the
catalytically active species we studied the gas-phase reaction
of the monoisotopic ion 5 of m/z 235 (selected in the
quadrupole) in the collision cell of the Q-TOF with ethene
and carried out mass analysis of the product ions using the
TOF analyzer.[23–25] Under low-energy collisions (1.1–2.0 V) in
the collision cell, the mass-selected and relatively cold cation
5 (which was quenched by low-energy collisions with the
neutral species) reacted with C2H4 yielding the cationic
addition product ions 7 (n = 1–4) in good yields (Figure 4 a).[12, 26, 27]
In the second set of experiments, we examined the
polymerization of ethene by mixing a toluene solution of
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2006, 45, 977 –981
Angewandte
Chemie
Figure 5. Ziegler–Natta polymerization of ethene: Positive-mode ESI mass spectrum of the
MeCN-quenched reaction solution of [Cp2ZrCl2]/MAO (1:1.2 equiv) and C2H4 (see Figure 1).
The spectrum shows the odd- and even-numbered-chain {Cp2Zr}–alkyl species, as well as the
MeCN adducts. The overall reaction time was approximately 1.7 s. The ion marked with (&) is
a Zr dimer impurity.
Figure 4. Proof of catalytic activity in the gas phase: MS/MS
product-ion mass spectra for reactions of ethene with a) 5,
b) 7 (n = 1), c) 7 (n = 20), d) 10 (n = 16), and e) 7·MeCN
(n = 21).
[Cp2ZrCl2] and MAO with a saturated solution of ethene in
toluene. The reacting solution was quenched with MeCN
(Figure 1).[28] In the resulting spectrum (Figure 5) four series
Angew. Chem. Int. Ed. 2006, 45, 977 –981
of ions can be observed: the odd-numbered-chain species 7
(n = 1–31) as well as 7·MeCN (n = 1–31) and the evennumbered-chain ions 10 (n = 8–31) as well as 10·MeCN (n =
7–29). It is known that in the solvent MeCN no polymerization activity is observed.[29] We rationalize that the dilution
step, which was approximately 1.7 s after the initiation of the
reaction of 5 and C2H4 in toluene, quenched all catalytically
active intermediates[30] by blocking the
empty d0 Zr orbital with solvent,[29] as
depicted in Figure 6.
Several ions found in this experiment were characterized further in
collision-induced dissociation (CID)
experiments. For example, ion 7 (n =
1) of m/z 263 shows a loss of H2 to give
Figure 6. MeCN deactivates the cationic Zr
the ion of m/z 261 ([Cp2Zr(allyl)]+) as
species by blocking
the main fragmentation pathway (Figthe free coordination
ure 3 c).[31] Similar behavior in the CID
site.
experiment was observed for most
[20]
ions 7 and 10.
Ions with higher
alkyl groups, i.e. 7 (n = 20) of m/z
796, undergo two consecutive losses of H2 affording, in this
case, the fragment ions of m/z 794 and 792, respectively
(Figure 3 d). The MeCN-adduct ions follow similar dissociative behavior after loss of the solvent molecule. As an
example, the ion 7·MeCN (n = 21) of m/z 865 undergoes loss
of MeCN to give the naked fragment ion 7 (n = 21) of m/z 824,
which further dissociates by two consecutive losses of H2
(spectrum not shown).
All the ions 7 and 10 should be catalytically active,
whereas the acetonitrile-adduct ions should not. This was
proved directly by investigating their reaction with ethene in
the gas phase. Ion 7 (n = 1) of m/z 263 (the most critical
compound for H2 loss)[12] showed the addition of four units of
ethene to give the product ions of m/z 291, 319, 347, and 375
(Figure 4 b). The ions 7 (n = 3, 10, 20) obtained in solution
showed that these species are also reactive toward C2H4, as
exemplified for ion 7 (n = 20) in Figure 4 c. The even-
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numbered chain species 10 (n = 16) of m/z 669 was also
reactive toward C2H4 showing two insertion products of m/z
697 and 725 (Figure 4 d). Finally, reaction of 7·MeCN (n = 21)
of m/z 865 is interesting since a loss of MeCN is initially
accomplished to give the naked cation 7 (n = 21) of m/z 824,
which is catalytically active, and ethene insertion can occur to
give the product ion of m/z 852 (Figure 4 e).[29]
In conclusion, cation [Cp2ZrCH3]+ (5) was easily detected
in the reaction solution of [Cp2ZrCl2] and MAO by ESI-MS, it
was characterized by MS/MS, and the catalytic activity was
directly demonstrated by the ion/molecule reaction of 5 and
ethene in the gas phase. Furthermore, for the first time, using
a microreactor coupled on-line to the ESI source we have
been able to intercept the intermediate alkyl zirconium
cations 7 of the growing polymer chain in the homogeneous
Ziegler–Natta polymerization of ethene directly from the
solution, characterize them mass spectrometrically, and prove
directly their catalytic activity by gas-phase reactions with
ethene.
Received: September 16, 2005
Published online: December 30, 2005
[11]
[12]
[13]
[14]
[15]
.
Keywords: homogeneous catalysis · mass spectrometry ·
microreactors · polymerization · reaction mechanisms
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A mixture of [Cp2ZrCl2] (17.2 mmol, 5.0 mg) and MAO
(20.6 mmol, 34.0 mL, 10 % solution in toluene) in anhydrous
and degassed toluene (5.0 mL) was mixed with anhydrous and
degassed MeCN under Ar atmosphere using a dual-syringe
pump operating at a flow rate of 5 mL min 1 in an effective
micromixer that was coupled directly to the ion source of the
mass spectrometer (Figure 2), and the solution was fed continuously into the MS. The flow rate could be varied from 2.5 to
100 mL min 1. By connecting the microreactor directly to the
spray capillary, reaction times from 0.3 to 28 s could be covered.
ESI-MS and ESI-MS/MS analyses were conducted in a highresolution hybrid quadrupole (Q) and orthogonal time-of-flight
(TOF) mass spectrometer (Q-TOF Premier, Micromass, UK)
with a constant nebulizer temperature of 150 8C. The ESI source
and the mass spectrometer were operated in the positive-ion
mode, and the cone and extractor potentials were set to 10 and
4.5 V, respectively, with a scan range of m/z 200–3000. Samples
were directly infused into the ESI source at flow rates of 5–
100 mL min 1 by means of a microsyringe pump. MS/MS experiments were carried out by mass selection of a specific ion in Q1
which was then submitted to collision-induced dissociation
(CID) with argon in the collision chamber. The product-ion
MS analysis was accomplished with the high-resolution orthogonal TOF analyzer.
Because zirconium displays five isotopes—90Zr (51.45 %), 91Zr
(11.22 %), 92Zr (17.15 %), 94Zr (17.38 %), and 96Zr (2.80 %)— the
Zr-containing species were be mass detected as clusters of
isotopologous ions.
Similar CID behavior was described in Ref. [12] and by C. S.
Christ, Jr., J. R. Eyler, D. E. Richardson, J. Am. Chem. Soc.
1990, 112, 596 – 607.
Although P. Chen and co-workers[12] reported that high voltages
were necessary to fragment similar clusters obtained through
borates, we observed ready release of MeCN.
It was reported that 5 reacts with MeCN to afford a stable
product of m/z 276 [Cp2Zr N=C(CH3)2]+ (Y. W. Alelyunas, R. F.
Jordan, S. F. Echols, S. L. Borkowsky, P. K. Bradley, Organometallics 1991, 10, 1406 – 1416). We performed the same reaction
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Chemie
[23]
[24]
[25]
[26]
[27]
[28]
[29]
[30]
[31]
as described by Jordan but employed MAO as co-catalyst, and
even after 48 h no product arising from reaction of 5 and MeCN
was observed by CID experiments.
The T-wave collision cell in the Q-TOF Premier used, enabled us
to observe several product ions: K. Giles, S. D. Pringle, K. R.
Worthington, D. Little, J. L. Wildgoose, R. H. Bateman, Rapid
Commun. Mass Spectrom. 2004, 18, 2401 – 2414.
The argon in the collision cell was changed for highly pure C2H4.
The ethene pressure used was between 3 O 10 3 and 2 O
10 2 mbar. For uses of QqTOFs, see: I. V. Chernushevich,
A. V. Loboda, B. A. Thomson, J. Mass Spectrom. 2001, 36,
849 – 865.
The product-ion mass spectrum was acquired under multiplecollision conditions that caused typical beam attenuations of 50–
70 % in the collision cell so as to increase the reaction yields and
to promote the collisional quenching of both the reactant and
product ions. Reactant ions should therefore display no or
negligible amounts of excess internal energy. For similar studies
in ion/molecule reactions, see: A. E. P. M. Sorrilha, L. S. Santos,
F. C. Gozzo, R. Sparrapan, R. Augusti, M. N. Eberlin, J. Phys.
Chem. A 2004, 108, 7009 – 7020, and references therein.
The gas-phase reaction with ion 5 demanded careful adjustment
of the voltages of the quadrupole, collision cell, and TOF
analyzer to prevent H2 loss giving allyl derivatives, which are not
catalytically active.[12,27] Although it was not possible to prevent
the H2 loss in this specific experiment completely, as depicted in
Figure 6 a, we observed the product ions of up to four C2H4
molecule insertions (n = 1–4) in good yields.
P. Watson, D. C. Roe, J. Am. Chem. Soc. 1982, 104, 6471 – 6473;
J. E. Bercaw, D. L. Davia, P. T. Wolczanski, Organometallics
1986, 5, 443 – 450.
A solution of 1 and MAO (Ref. [17]) in toluene was mixed in an
effective micromixer with toluene saturated with ethene by using
a dual-syringe pump set at one flow rate. The reacting solution
was delivered to the second micromixer to accomplish the
dilution with anhydrous and degassed MeCN, under Ar atmosphere. The second micromixer was coupled directly to the ion
source of the mass spectrometer (Figure 1), and the solution was
fed continuously into the MS. The reaction time with this device
is estimated to be around 0.3 s (50 mL min 1) to 1.7 s (5 mL min 1)
depending on the velocity of the syringe pump used to deliver
the compounds to the second micromixer for MeCN dilution.
Micromixer (MR1): capillary 75 mm (ID), l1 = 3 cm, V = 2.2 O
10 10 m3 ; Micromixer (MR2): capillary 75 mm (ID), l2 = 5 cm.
a) R. F. Jordan, W. E. Dasher, S. F. Echols, J. Am. Chem. Soc.
1986, 108, 1718 – 1719; b) R. F. Jordan, C. S. Bajgur, R. Willett, B.
Scott, J. Am. Chem. Soc. 1986, 108, 7410 – 7411.
Varying the length of the PEEK from the second micromixer
(l2), which brings the reaction solution after MeCN dilution to
ESI ion source, did not affect the relative intensities of the
observed ions (Figure 1, bottom right).
D. B. Jacobson, B. S. Freiser, J. Am. Chem. Soc. 1983, 105, 736 –
742.
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