Catalyst/cocatalyst nuclearity effects in single-site olefin polymerization. Significantly enhanced 1-octene and isobutene comonomer enchainment in ethylene polymerizations mediated by binuclear catalysts and cocatalysts

Article abstract of DOI:10.1021/ja036289c

This Communication describes the implementation of a new binuclear homometallic organotitanium constrained geometry catalyst (CGC), (μ-CH₂CH₂-3,3′){ (η⁵-indenyl )[1-Me₂Si (^tBuN)](TiMe₂)}₂[EBICGC(TiMe₂)₂; Ti₂], together with the bifunctional activators (Ph₃C⁺)₂[1,4-(C₆F₅)₃BC₆F₄B(C₆F₅)₃]^2- (B₂) and new bisborane 1,4-(C₆F₅)₂BC₆F₄B(C₆F₅)₂ (BN₂) in ethylene + α-olefin copolymerization processes. Specifically examined are the comonomers 1-octene and poorly responsive isobutene. Large increases in comonomer enchainment efficiency into the polyethylene microstructure are observed versus the corresponding mononuclear catalyst [1-Me₂Si(3-ethylindenyl)(^tBuN)]TiMe₂ (Ti₁) + Ph₃C⁺B(C₆F₅)₄^- (B₁) or B(C₆F₅)₃ (BN) under identical polymerization conditions. In ethylene + 1-octene copolymerization, 11 times more 1-octene incorporation is observed for Ti₂ + B₂ vs Ti₁ + B₁. In ethylene + isobutene copolymerization, 5 times more isobutene incorporation is observed for Ti₂ + BN₂ vs Ti₁ + BN. Copyright

Full text of DOI:10.1021/ja036289c

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Catalyst/Cocatalyst Nuclearity Effects in Single-Site Olefin Polymerization.
Significantly Enhanced 1-Octene and Isobutene Comonomer Enchainment in
Ethylene Polymerizations Mediated by Binuclear Catalysts and Cocatalysts
Hongbo Li,† Liting Li,† Tobin J. Marks,*,† Louise Liable-Sands,‡ and Arnold L. Rheingold‡
Department of Chemistry, Northwestern UniVersity, EVanston, Illinois 60208-3113, and Department of Chemistry
and Biochemistry, UniVersity of Delaware, Newark, Delaware 19716-2522
Received May 22, 2003; E-mail: t-marks@northwestern.edu
Scheme 1. Synthetic Routes to Binuclear Catalyst and Cocatalyst
Enzymatic catalysts achieve superior reactivity and selectivity,
in part due to high local reagent concentrations and conformationally
advantageous spatial proximities/interactions.1 In this regard, the
possibility of unique/more efficient catalytic transformations based
on cooperative effects between active centers in multinuclear
complexes is being intensively investigated.2 For single-site olefin
polymerization catalysts,3-5 two connectivity strategies (electrostatic
and covalent) for bringing catalyst centers into spatial proximity
have been pursued to achieve cooperative effects via multinuclear
“CGC” structures.4a,b Regarding electrostatic approaches, it was
recently shown4a that bisborate cocatalyst B2 dramatically increases
TiMe2 (Ti1) was synthesized in a similar manner for control
experiments.8 Binuclear bisborane activator 1,4-(C6F5)2BC6F4B-
(C6F5)2 (BN2) was synthesized via reaction of 1,4-Me3SnC6F4SnMe3
and excess (C6F5)2BCl (Scheme 1B). All new compounds were
characterized by standard spectroscopic/analytical/diffraction
methodologies.8
As benchmarks, ethylene + 1-octene copolymerizations with four
combinations of CGCTi catalysts and cocatalysts of varying
nuclearity (Ti1 + B1, Ti1 + B2, Ti2 + B1, Ti2 + B2) were first
examined under identical anhydrous/anaerobic reaction conditions.4a,b
The polymerization data (Table 1) indicate that, under identical
conditions, the catalyst derived from bimetallic Ti2 and bifunctional
cocatalyst B2 enchains ∼11 times more 1-octene9 than that derived
from mononuclear Ti1 and B1, while Ti1 + B2 and Ti2 + B1 each
incorporate ∼2 times more. The activity of these CGCTi catalysts
activated by the trityl borates is ∼100 times greater, and the product
molecular weight is ∼100 times greater than previously achieved
with the Zr analogues.4b
Efficient isobutene + ethylene coordinative copolymerization
presents a daunting challenge from both academic and technological
perspectives.10a,b Recently, ethylene + isobutylene copolymers with
moderate comonomer contents were prepared using modified
mononuclear CGCTi catalysts and Very large isobutene stoichio-
metric excesses (isobutene:ethylene up to 150:1).10c In the present
work, ethylene + isobutene polymerization was examined with
several CGC catalysts and cocatalysts of varying nuclearity. It is
found that CGCZr catalysts (Zr1 or Zr2) in combination with any
of the cocatalysts introduce negligible quantities of the isobutene
comonomer. With Ti, the results are sensitive to cocatalyst. Trityl
borate activators (B1 or B2) alone initiate cationic isobutene
homopolymerization,10d affording physical mixtures of polyethylene
and polyisobutene homopolymers in the presence of Ti1 or Ti2.
MAO avoids this problem but yields copolymers with low activity.
With MAO, Ti2 incorporates ∼2 times more isobutene than Ti1.
Next, polymerizations were carried out with bisborane cocatalyst
BN2, affording ethylene-isobutene copolymers with reasonable
the efficiency of heterobimetallic enchainment processes by bringing
cationic Me2Si(tBuN)(η5-C5Me4)TiMe+ (producing high-molecular-
weight polyethylene with high activity) and 1-Me2Si(3-ethylin-
denyl)(tBuN)]ZrMe+ (producing low-molecular-weight polyethylene
with low activity, Zr1) centers into proximity, thereby increasing
selectivity for branched LLDPE formation. Regarding covalent
4b,6
approaches, it was recently reported4b that bimetallic Zr24b + B2
affords enhanced branching in ethylene homopolymerization and
enhanced comonomer incorporation in ethylene + 1-pentene
copolymerization vs Zr1 + B1. Nevertheless, CGCZr catalysts
produce low-molecular-weight polyolefins with low activity and
low R-olefin coenchainment efficiency,4b,7 raising the intriguing
question of what properties bimetallic CGCTi catalysts might
exhibit, since mononuclear CGCTi catalysts typically produce high-
molecular-weight polyolefins with high activity and high R-olefin
coenchainment efficiency.4a,5 Here we report that a new binuclear
organotitanium catalyst + binuclear activators affords, vs mono-
nuclear analogues, high-molecular-weight polyethylenes with sig-
nificantly enhanced R-olefin comonomer incorporation and, in
particular, significantly enhanced incorporation of traditionally
unreactive isobutene.
Bimetallic catalyst precursor EBICGC(TiMe2)2 (Ti2) was syn-
thesized by Al2(CH3)6 alkylation of EBICGC[Ti(NMe2)2]2 (Scheme
1A),8 while monometallic [1-Me2Si(3-CH2CH3indenyl)(tBuN)]-
† Northwestern University.
‡ University of Delaware.
9
10788
J. AM. CHEM. SOC. 2003, 125, 10788-10789
10.1021/ja036289c CCC: $25.00 © 2003 American Chemical Society

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C O M M U N I C A T I O N S
Table 1. Ethylene + R-Olefin Copolymerization Results for Catalysts Ti2, Ti1 and Cocatalysts B2, B1, BN2, BN
comonomer
concn (M)
µmol of
cat.
reaction
time (min)
polymer
yield (g)
comonomer
entry
cat.
cocat.
B1
B2
B1
B2
BN
BN2
BN
BN2
MAOd
MAOd
comonomer
activityb
10-3 M c
M /M c
incorporation (%)e
w
w
n
1
2
3
4
5
6
7
8
9
10
Ti1
Ti1
Ti2
Ti2
Ti1
Ti1
Ti2
Ti2
Ti1
Ti2
1-octene
1-octene
1-octene
1-octene
isobutene
isobutene
isobutene
isobutene
isobutene
isobutene
0.64
0.64
0.64
0.64
1.2
1.2
1.2
1.2
1.2
10
10
5
5
5
5
5
5
8.43
3.20
4.30
2.50
0.80
0.37
0.61
0.47
0.39
0.44
1.0 × 107
3.9 × 106
5.1 × 106
3.0 × 106
9.6 × 105
4.4 × 105
3.6 × 105
2.8 × 105
3.9 × 104
1.1 × 104
155
147
157
161
577
305
490
168
487
355
2.33
1.99
2.75
2.73
2.13
2.16
2.41
3.67
2.52
2.87
0.6
1.1
1.0
7.0
3.1
9.5
7.3
15.2
2.9
6.2
5
10
10
5
5
10
5
5
10
10
60
240
1.2
a Polymerizations carried out on a high-vacuum line at 24 °C in 100 mL of toluene under 1.0 atm ethylene pressure. b Gram polymer/[(mol cationic
metallocene)‚atm‚h]. c From GPC vs polystyrene standards; all distributions are monomodal. d Al:Ti ) 1000:1. e For ethylene +1-octene copolymer, calculated
from 13C NMR spectra;9a,b for ethylene + isobutene copolymer, calculated from 13C NMR spectra.10c
activity (Table 1).11 Importantly, under identical, stoichiometrically
(3) For recent reviews of single-site olefin polymerization, see: (a) Gibson,
V. C.; Spitzmesser. S. K. Chem. ReV. 2003, 103, 283-316. (b) Gladysz,
conservative polymerization conditions (isobutene:ethylene ) 8.8:
J. A., Ed. Chem. ReV. 2000, 100 (special issue on “Frontiers in Metal-
Catalyzed Polymerization”). (c) Marks, T. J., Stevens, J. C., Eds. Topics
1), binuclear Ti2 + bifunctional BN2 incorporates ∼5 times more
Catal. 1999, 15, and references therein.
isobutene than the mononuclear analogues (∼30-fold increase in
(4) For studies of binuclear metallocenes, see: (a) Abramo, G. P.; Li, L.;
isobutene:ethylene reactivity ratio), while product molecular weight
and polymerization activities decline only moderately with increased
catalyst/cocatalyst nuclearity. 13C NMR analysis of the copolymer
microstructure8 reveals that most enchained isobutenes are separated
by two or more ethylene units, with small quantities separated by
a single ethylene unit. The 1H spectrum reveals vinylidene
endgroups derived from â-methyl elimination. Steric congestion is
known to play an important role in metallocenium â-methyl
elimination processes.10d,e,12 The present results show that the
sterically open CGC structure favors ethylene + isobutene co-
propagation by decreasing the relative rate of chain termination.
In conclusion, new types of binuclear CGCTi catalysts and
bifunctional activators have been investigated. They exhibit greatly
enhanced polymerization activity, polyolefin molecular weight, and
comonomer incorporation efficiency vs the Zr analogues. Particu-
larly noteworthy is the increased selectivity for highly encumbered
comonomer enchainment, presumably facilitated via cooperative
comonomer capture/binding/delivery by the proximate cationic
centers.4a,b,13
Marks, T. J. J. Am. Chem. Soc. 2002, 124, 13966-13967. (b) Li, L.; Metz,
M. V.; Li, H.; Chen, M.-C.; Marks, T. J.; Liable-Sands, L.; Rheingold,
A. L. J. Am. Chem. Soc. 2002, 124, 12725-12741. (c) Spaleck, W.; Kuber,
F.; Bachmann, B.; Fritze, C.; Winter, A. J. Mol. Catal. A: Chem. 1998,
128, 279-287.
(5) (a) Chum, P. S.; Kruper, W. J.; Guest, M. J. AdV. Mater. 2000, 12, 1759-
1767. (b) Lai, S. Y.; Wilson, J. R.; Knight, G. W.; Stevens, J. C. Int.
Patent WO-93/08221, 1993.
(6) Similar bifunctional cocatalysts: (a) Metz, M. V.; Schwartz, D. J.; Stern,
C. L.; Marks, T. J.; Nickias, P. N. Organometallics 2002, 21, 4159-
4168. (b) McAdon, M. H.; Nickias, P. N.; Marks, T. J.; Schwartz, D. J.
Int. Patent WO-99/06413A1, Feb 11, 1999. (c) Williams, V. C.; Piers,
W. E.; Clegg, W.; Elsegood, M. R. J.; Collins, S.; Marder, T. B. J. Am.
Chem. Soc. 1999, 121, 3244-3245.
(7) Jia, L.; Yang, X.; Stern, C. L.; Marks, T. J. Organometallics 1997, 16,
842-857.
(8) See Supporting Information for details.
(9) NMR assay: (a) Liu, W.; Ray, D. G., III; Rinaldi, P. L. Macromolecules
1999, 32, 3817-3819. (b) We assume most long-chain branches in the
ethylene + 1-octene copolymer are 1-octene-derived n-hexyl branches
since negligible branching is detected in the ethylene homopolymer under
these polymerization conditions.
(10) (a) Pino, P.; Giannini, U.; Porri, L. In Encyclopedia of Polymer Science
and Engineering, 2nd ed.; Mark, H. F., Bikales, N. M., Overberger, C.
C., Menges, G., Eds.; Wiley: Interscience: New York, 1987; Vol. 8, p
175. (b) Kaminsky, W.; Bark, A.; Spiehl, R.; Mo¨ller-Linderhof, N.;
Niedoba, S. In Transition Metals and Organometallics as Catalysts for
Olefin Polymerization; Kaminsky, W., Sinn, H., Eds.; Springer-Verlag:
Berlin, 1988; pp 291f. (c) Shaffer, T. D.; Canich, J. A. M.; Squire, K. R.
Macromolecules 1998, 31, 5145-5147. Differences in reported experi-
mental procedures and reaction conditions prevent a meaningful com-
parison of monomer reactivity ratios with the present results. (d) Shaffer,
T. D.; Ashbaugh, J. R. J. Polym. Sci., A: Polym. Chem. 1997, 35, 329-
331. (e) Horton, A. D. Organometallics 1996, 15, 2675-2677.
(11) (a) B(C6F5)3 does not initiate cationic isobutene polymerization in
toluene,11c and the present copolymerizations with ethylene are inconsistent
with a cationic pathway.11b,c (b) Baird, M. C. Chem. ReV. 2000, 100, 1471-
1478 and references therein. (c) Barsan, F.; Karam, A. R.; Parent, M. A.;
Baird, M. C. Macromolecules 1998, 31, 8439-8447.
Acknowledgment. The research was supported by DOE
(86ER13511) and NSF (CHE0078998). L.L. thanks Dow Chemical
for a postdoctoral fellowship.
Supporting Information Available: Details of catalyst, cocatalyst
syntheses, polymerization experiments, and crystal structures (PDF,
CIF). This material is available free of charge via the Internet at
http://pubs.acs.org.
(12) (a) Beswick, C. L.; Marks, T. J. J. Am. Chem. Soc. 2000, 122, 10358-
10370. (b) Chirik, P. J.; Day, M. W.; Labinger, J. A.; Bercaw, J. E. J.
Am. Chem. Soc. 1999, 121, 10308-10317.
References
(1) See, for example: (a) Krishnan, R.; Voo, J. K.; Riordan, C. G.; Zahkarov,
L.; Rheingold, A. L. J. Am. Chem. Soc. 2003, 125, 4422-4423. (b) Bruice,
T. C. Acc. Chem. Res. 2002, 35, 139-148.
(2) See, for example: (a) Trost, B. M.; Mino, T. J. Am. Chem. Soc. 2003,
125, 2410-2411. (b) Jacobsen, E. N. Acc. Chem. Res. 2000, 33, 421-
431. (c) Molenveld, P.; Engbersen, J. F. J.; Reinhoudt, D. N. Chem. Soc.
ReV. 2000, 29, 75.
(13) In related ethylene polymerization work to be published elsewhere, we
4b
find that the -CH2- bridged analogue of -CH2CH2- bridged Zr2
exhibits enhanced selectivity for 1-hexene incorporation vs Zr2, doubtless
a consequence of the closer Zr-Zr spatial proximity (Li, H.; Li, L.; Marks,
T. J., unpublished observations).
JA036289C
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J. AM. CHEM. SOC. VOL. 125, NO. 36, 2003 10789