Bulky aluminum alkyl scavengers in olefin polymerization with group 4 catalysts

Article abstract of DOI:10.1021/ja030121+

The binding of H₂O to MeAl(OAr)₂ (1: Ar = 2,6-di-tert-butyl-4-methylphenyl) in THF-d₈ at -40 °C provides aquo complex 2, the structure of which was determined by X-ray crystallography. Complex 2 is unstable above 0 °C in THF-d₈ and decomposes to form ArOH (major), CH₄ (minor), and a methyl aluminoxane of undetermined structure. Decomposition of 2 follows first-order kinetics with k = 3.0 × 10^-4 s^-1 at 5 °C. The hindered phenol ArOH slowly reacts with [Cp₂ZrMe][MeB(C₆F₅)₃] (4) in bromobenzene-d₅ solution at 25 °C to furnish CH₄ and [Cp₂ZrOAr][MeB(C₆F₅)₃] (5), the structure of which was confirmed by X-ray crystallography. This reaction follows second-order kinetics for [ArOH] = [4] = 0.045 M and with k = 2.8 × 10^-3 M^-1 s^-1 at 25 °C. This corresponds to a rate that is >10⁷ × slower than the apparent rate of ethylene insertion for 4 at 25 °C at typical concentrations encountered in olefin polymerization. The kinetic data, as well as control experiments involving the addition of ArOH to active catalyst producing poly(ethylene), demonstrate that ArOH has essentially no effect on polymerization kinetics involving 4. Copyright

Full text of DOI:10.1021/ja030121+

Published on Web 07/12/2003
Bulky Aluminum Alkyl Scavengers in Olefin Polymerization with Group 4
Catalysts
Russell A. Stapleton,† Brandon R. Galan,† Scott Collins,*,† Richard S. Simons,‡
Jered C. Garrison,‡ and Wiley J. Youngs‡,§
Department of Polymer Science, UniVersity of Akron, Akron, Ohio 44325, and
Department of Chemistry, UniVersity of Akron, Akron, Ohio 44325
Received February 20, 2003; E-mail: collins@uakron.edu
The study of olefin polymerization using cationic, transition-
metal complexes has been facilitated through the design and study
of discrete activators that provide well-defined ion pairs.1 Typically
an aluminum alkyl scavenger is used with these activators and the
catalyst precursor to avoid adventitious catalyst deactivation, at least
in a commercial setting.2 As is now known, aluminum alkyls
(AlMe3, AliBu3, etc.) are often reactive toward these activators or
the ion pairs formed from them.3 In addition, they can reversibly
inhibit catalysts or act as chain transfer agents, thus modifying
intrinsic behavior.4 We have reported over the past few years that
MeAl(OAr)2 (1: Ar ) 2,6-di-tert-butyl-4-methylphenyl),5 is a “non-
interacting” scavenger; it does not react readily with common
activators and ion pairs derived from them.6,7
We noted that an excess of 1 was needed to remove impurities
from monomer and solvent, compared to more reactive aluminum
alkyls (for an impurity level of, for example, 50 µM H2O, 250-
500 µM 1 vs 50 µM of AlMe3 would be required). We attributed
this to slower reaction of 1 with H2O to produce CH4, the expected
hydrolysis byproduct of MeAlX2 compounds.8 We now report that
the reaction of 1 with H2O is slow, and that, although CH4 is
formed, the primary product of hydrolysis is 2,6-di-tert-butyl-4-
methylphenol (BHT).
12a
is similar to that observed in Mes3Ga(OH2)‚THF2 and (Ph3-
SiO)3Al(OH2)‚THF2.12b
Crystals of 2 are unstable at 25 °C, losing THF over a period of
several hours under N2. In THF-d8 solution, 2 is unstable above 0
°C, and H NMR spectra reveal formation of BHT (1.5 equiv per
1
equiv of 2), CH4 (0.10 equiv) and oddly enough, compound 1 (0.20
equiv).13 In addition, a broad signal grows in at δ -0.78 due to an
Al-Me group (0.70 equiv); this resonance resembles that of
methylaluminoxane in this solvent,14 suggesting a similar oligomeric
structure for this product.
The decomposition of 2 follows 1st order kinetics with kobs
)
The reaction between H2O and 1 in toluene solution was
complicated by the low solubility of H2O and the more rapid nature
of the hydrolysis reaction in this solvent. Both CH4 and BHT were
formed, but we were unable to control/monitor the extent of
reaction. In THF-d8, binding of H2O to 1 could be observed and
3.0 × 10-4 s-1 at 5 °C. That BHT is the sole 1° product while CH4
and additional BHT are formed in subsequent steps, was revealed
by monitoring the decomposition of the analogous MeOH adduct
which liberated 1 equiv of BHT (and no CH4) under these conditions
(see Supporting Information).15
1
the kinetics of hydrolysis monitored by H NMR spectroscopy.9
Addition of 1.0 equiv of H2O to a THF-d8 solution of 1 below
-40 °C provides the aquo complex 2 (eq 1). The NMR spectro-
scopic data (see Supporting Information), in particular the 1H
resonance due to the aquo protons at δ 9.71 and the upfield shift
of the Al-Me resonance to δ -0.49, are consistent with spectra
of aquo complexes of hindered trialkyl- and triaryl-aluminum
compounds.8
The molecular structure of complex 2 is depicted in eq 1;10 this
is the first structurally characterized aquo complex of an aluminum
alkyl. Notable features of this structure include strong H-bonding
between the coordinated H2O and the two THF molecules with
H(3OA)-O(4) and H(3OB)-O(5) distances of 1.79(4) and 1.68(6)
Å, respectively and shortened Al-O distances for the phenoxy
groups [av 1.718(3) Å] compared with the Al-O(3) distance of
1.863(3) Å for the formally dative bond involving coordinated H2O.
The geometry about Al is similar to that observed in HAl(OAr)2‚
OEt2,11 while the H-bonding between H2O and the THF molecules
Proton transfer to OAr and loss of BHT is more facile than loss
of CH4, despite the driving force for forming an Al-O bond at the
expense of a weaker Al-C bond. Similar conclusions may be
t
inferred from the hydrolyses of donor-stabilized Bu2Al[N(Me)-
CH2CH2NMe2]16a or [tBu2Al(µ-OC6H4O)-AltBu2]n in the presence
of 2,6-lutidine;16b results of this type have been interpreted as arising
from the reduced basicity of the Al-C bonds in such compounds.17
However, in the controlled hydrolysis of the alane, [HAl(OAr)2],
H2 is exclusively formed.11b We suspect the difference in chemo-
selectivity observed in the hydrolysis of Al-R vs Al-H in these
hindered compounds may reflect factors other than differences in
basicity.
As BHT is the major hydrolysis product formed from 1, C2H4
polymerization experiments using [Cp2ZrMe][MeB(C6F5)3] [4,
generated in situ from Cp2ZrMe2 and B(C6F5)3] where solvent and
monomer were pretreated with excess 1, must have been conducted
in the presence of significant quantities of BHT (i.e. ∼150 µM
BHT for an impurity level of 100 µM, expressed as H2O).6a The
implication is that this phenol is not very reactive toward metal-
locenium ions! To investigate this hypothesis, we examined the
reaction of 4 [generated in situ from Cp2ZrMe2 and B(C6F5)3,] with
† Department of Polymer Science.
‡ Department of Chemistry.
§ Author to whom correspondence should be addressed regarding the X-ray
structure of 2 and 5.
9
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J. AM. CHEM. SOC. 2003, 125, 9246-9247
10.1021/ja030121+ CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
BHT in bromobenzene-d5 (BB-d5) solution in which the product
of this reaction (5) is soluble. As illustrated in eq 2, the novel,
Nicholas J. Taylor and some exploratory experiments performed
by Muqtar Mohammed at the University of Waterloo.
phenoxide complex 5 and CH4 are slowly formed at 25 °C (t1/2
2 h at [4]o ) [BHT]o ) 0.045 M).
∼
Supporting Information Available: Experimental details for the
syntheses of compounds 1-5, polymerization procedures, and X-ray
crystallographic, refinement and metrical data for compounds 2 and 5
(PDF and CIF). This material is available free of charge via the Internet
at http://pubs.acs.org
References
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The molecular structure of 5 appears in eq 2.18 The structure of
5 consists of separated ions in contrast to many structures featuring
the [Me(B(C6F5)3] anion.1 In BB-d5 solution, there also appears to
be limited interaction of the counteranion with Zr as judged from
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1
the H and 19F NMR spectra (see Supporting Information) which
are characteristic of “free” [MeB(C6F5)3] in this solvent.19 Details
of this structure will be reported elsewhere.
The reaction of 4 with BHT exhibits second-order kinetics at
[4]o ) [BHT]o ) 0.045 M with an observed rate constant of 2.8 ×
10-3 M-1 s-1 at 25 °C (see Supporting Information). This
corresponds to a rate that is about 107 times slower than the apparent
rate of ethylene insertion (in toluene at 25 °C),6a at typical
concentrations in a reactor (i.e. [BHT] e 1 mM, [C2H4] g 100
mM).
(8) For a review on the hydrolysis of organoalanes, see: Roesky, H. W.;
Walawalkar, M. G.; Murugavel, R. Acc. Chem. Res. 2001, 34, 201-211.
(9) The product distribution is similar in THF vs that in aromatic solvents.
(10) Structure of compound 2 with 30% thermal ellipsoids depicted. Only H
atoms of interest are shown. Single crystals of 2 were obtained by layering
a concentrated THF solution with hexane at low temperature. Crystal-
lographic data (100 K): Monoclinic, space group Cc, a ) 15.2211(13)
Å, b ) 16.7155(14) Å, c ) 15.1522(13) Å, â ) 98.436(2)°, V ) 3813.4(6)
Å3, Z ) 4, RF ) 0.0611, RwF ) 0.1104 for 6493 unique reflections with
I > 2σ(I). Selected bond lengths (Å) and angles (deg) with estimated
standard deviation in parentheses: Al-O(3) ) 1.863(3); Al-O(2) )
1.715(3); Al-O(1) ) 1.721(3); Al-C(31) ) 1.953(5); Al-O(1)-C(1)
) 169.6(2); Al-O(2)-C(16) ) 171.3(2); O(3)-Al-C(31) ) 102.0(2).
(11) (a) Healy, M. D.; Power, M. B.; Barron, A. R. Coord. Chem. ReV., 1994,
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(b) Apblett, A. W.; Warren, A. C.; Barrron, A. R. Can. J. Chem. 1992,
70, 771-78.
Figure 1. Simulated scavenging of H2O by MeAl(BHT)2 (1). Initial
concentrations are 60 µM H2O, and 600 µM 1. Inset: Decomposition of
[Cp2ZrMe][MeB(C6F5)3] (4: [Zr]o ) 4 µM) by reaction with BHT.
(13) We suspect compound 1 forms because a more Lewis acidic or reactive
intermediate competes for water with aquo complex 2.
This is illustrated in Figure 1 where the disappearance of 2 and
formation of BHT is simulated from the kinetic data while the rate
of decomposition of 4 through reaction with BHT is also shown
(inset) to illustrate the dramatically different time scales! Further,
the deliberate addition of BHT (ca. 250 equiv with respect to 4)
during ethylene polymerization initiated by complex 4 at 25 °C
and 28 psi had no effect on the rate of polymerization (see
Supporting Information).
It is clear from these results that 1 is useful as a scavenger,
provided that the active catalyst (or cocatalyst) is not reactive toward
BHT. This certainly seems to be the case for a number of group 4
catalysts or Lewis acidic cocatalysts under mild conditions.6
(14) Imhoff, D. W.; Simeral, L. S.; Sangokoya, S. A.; Peel, J. H. Organome-
tallics 1998, 17, 1941-1945.
(15) Similar results have been reported for the reaction of iPrOH with 1. Taden,
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2000, 441-445.
(16) (a) van Poppel, L. G.; Bott, S. G.; Barron, A. R. J. Chem. Crystallogr.
2002, 31, 417-420. (b) McMahon, C. N.; Barron, A. R. J. Chem. Soc.,
Dalton Trans. 1998, 3703-04.
(17) (a) Healy, M. D.; Leman, J. T.; Barron, A. R. J. Am. Chem. Soc. 1991,
113, 2776-77. (b) Healy, M. D.; Ziller, J. W.; Barron, A. R. Organo-
metallics 1991, 10, 597-608.
(18) Structure of 5 with 50% thermal ellipsoids depicted. The [MeB(C6F5)3]
anion and Cp-H atoms are not shown. Single crystals of 5 were grown
by layering a concentrated solution in CH2Cl2 with hexane. Crystal-
lographic data (100 K): Monoclinic, space group P21/n, a ) 10.9075(5)
Å, b ) 30.7926(15) Å, c ) 11.7696(6) Å, â ) 97.8040(10)°, V )
3916.4(3) Å3, Z ) 4, RF ) 0.0279, RwF ) 0.0708 for 8937 unique
reflections with I > 2σ(I).
Acknowledgment. This work was supported by the University
of Akron. In addition, we acknowledge the support of the National
Science Foundation for the purchase of a CCD diffractometer (CHE-
0116041). Finally we acknowledge helpful discussions with Dr.
(19) Niehues, M.; Erker, G.; Kehr, G.; Schwab, P.; Frohlich, R.; Blacque, O.;
Berke, H. Organometallics 2002, 21, 2905-11.
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