Organotitanium-Mediated Ethylene Polymerization
A R T I C L E S
Scheme 1 . Proposed Catalytic Cycle for Organotitanium-Mediated
Ethylene Polymerization in the Presence of Electron-Deficient
Chain-Transfer Agentsa
Ti‚h‚atm ethylene) and to afford high molecular weight polymers
with narrow polydispersities. These “CGC” catalysts produce
polyethylene containing long-chain branches (LCBs) under
circumstances in which vinyl-terminated, chain-transferred
macromolecules have an elevated probability of re-enchainment
into the growing polymer chain at a second catalyst center.2
The resulting small but significant levels of long-chain branch-
ing, where we define a LCB as a polymeric branch, lead to
extremely advantageous materials properties.2 In addition, such
titanium catalysts can efficiently polymerize and copolymerize
sterically encumbered comonomers that have traditionally been
difficult to enchain.11,12 Polynuclear organotitanium catalysts,
such as (µ-CH2CH2-3,3′){(η5-indenyl)[1-Me2Si(tBuN)]}2Ti2Me4
(EBICGCTi2Me4, 2), also afford high molecular weight poly-
olefins with dramatically enhanced R-olefin comonomer incor-
poration versus mononuclear analogues.12 The dicationic bi-
metallic framework likely exhibits enhanced binding affinity
(e.g., 3), resulting in enhanced comonomer enchainment.
a P ) polymer chain; E - Si, B, Al; R ) alkyl, H; R′ ) alkyl, aryl.
introduce functionality at macromolecule chain ends.3-8 A priori,
introducing such functionality in concert with the polymerization
process is preferred over post-polymerization modification,
which can be difficult due to the unreactive nature of hydro-
carbon polymers and the lack of control in both macromolecule
functionality levels and locations. To date, strategies to introduce
comonomers that have the capacity to undergo rapid insertion
(chain propagation) as well as to effect reactive functionality-
introducing chain-transfer processes have not, to our knowledge,
been explored.9
Organotitanium complexes are among the most versatile
catalysts for single-site Ziegler-Natta-type R-olefin polymer-
ization.10 Organotitanium catalysts such as Me2Si(Me4C5)-
(NtBu)TiMe2 (CGCTiMe2, 1), in combination with appropriate
activators/cocatalysts, are known to effect the polymerization
of various R-olefins with activities as large as 107 g/(mol of
(3) Organoaluminum chain transfer: (a) Go¨tz, C.; Rau, A.; Luft, G. Macromol.
Mater. Eng. 2002, 287, 16. (b) Kukral, J.; Lehmus, P.; Klinga, M.; Leskela¨,
M.; Rieger, B. Eur. J. Inorg. Chem. 2002, 1349. (c) Han, C. J.; Lee, M. S.;
Byun, D.-J.; Kim, S. Y. Macromolecules 2002, 35, 8923. (d) Liu, J.;
Støvneng, J. A.; Rytter, E. J. Polym. Sci., Part A: Polym. Chem. 2001,
39, 3566.
(4) Silane chain transfer: (a) Amin, S. B.; Marks, T. J. J. Am. Chem. Soc.
2006, 128, 4506. (b) Koo, K.; Marks, T. J. J. Am. Chem. Soc. 1999, 121,
8791. (c) Koo, K.; Fu, P.-F.; Marks, T. J. Macromolecules 1999, 32, 981.
(d) Koo, K.; Marks, T. J. Chemtech 1999, 29 (10), 13. (e) Fu, P.-F.; Marks,
T. J. J. Am. Chem. Soc. 1995, 117, 10747.
(5) Borane chain transfer: (a) Dong, J. Y.; Wang, Z. M.; Hong, H.; Chung, T.
C. Macromolecules 2002, 35, 9352. (b) Dong, J. Y.; Chung, T. C.
Macromolecules 2002, 35, 1622. (c) Dong, J. Y.; Manias, E.; Chung, T.
C. Macromolecules 2002, 35, 3430. (d) Chung, T. C.; Dong, J. Y. J. Am.
Chem. Soc. 2001, 123, 4871.
(6) Thiophene chain transfer: Ringelberg, S. N.; Meetsma, A.; Hessen, B.;
Teuben, J. H. J. Am. Chem. Soc. 1999, 121, 6082.
(7) Phosphine chain transfer: (a) Kawaoka, A. M.; Marks, T. J. J. Am. Chem.
Soc. 2005, 127, 6311. (b) Kawaoka, A. M.; Marks, T. J. J. Am. Chem.
Soc. 2004, 126, 12764.
(8) Organozinc chain transfer: (a) Arriola, D. J.; Carnahan, E. M.; Hustad, P.
D.; Kuhlman, R. L.; Wenzel, T. T. Science 2006, 312, 714. (b) van Meurs,
M.; Britovsek, G. J. P.; Gibson, V. C.; Cohen, S. A. J. Am. Chem. Soc.
2005, 127, 9913 and references therein.
(9) For representative patent literature regarding olefin/alkenylsilane polym-
erization, see: (a) Arriola, D. J.; Bishop, M. T.; Campbell, R. E.; Devore,
D. D.; Hahn, S. E.; Ho, T. H.; McKeand, T. J.; Timmers, F. J. (Dow
Chemical Co.). U.S. Patent Application US 234831, 2003. (b) Sugimoto,
R.; Ishii, Y.; Kawanishi, K.; Nishimori, Y.; Inoe, N.; Asanuma, T. (Mitsui
Toatsu Chemical Co.). Japan Patent Application JP 0830281, 1996. (c)
Montecatini Chemical Co. U.S. Patent Application US 3644306, 1969.
(10) (a) Okuda, J. Dalton Trans. 2003, 12, 2367. (b) Metz, M. V.; Sun, Y.;
Stern, C. L.; Marks, T. J. Organometallics 2002, 21, 3691. (c) McKnight,
A. L.; Waymouth, R. M. Chem. ReV. 1998, 98, 2587. (d) Chen, Y.-X.;
Marks, T. J. Organometallics 1997, 16, 3649. (e) Stevens, J. C.; Timmers,
F. J.; Wilson, D. R.; Schmidt, G. F.; Nickias, P. N.; Rosen, R. K.; Knight,
G. W.; Lai, S. (Dow Chemical Co.). European Patent Application EP
0416815 A2, 1991; Chem. Abstr. 1991, 115, 93163.
In addition to these interesting characteristics, many single-
site organotitanium catalysts exhibit high activity for silanolytic
chain transfer,4b a process which efficiently introduces orga-
nosilane functionality into polyolefin chains (Scheme 1). These
observations raise the intriguing question of whether the two
types of transformations could be coupled by introducing
unsaturated alkenylsilanes as comonomers into a single-site
(11) Mononuclear organotitanium-mediated polymerization of functionalized and
sterically hindered monomers: (a) Yoon, J.; Mathers, R. T.; Coates, G.
W.; Thomas, E. L. Macromolecules 2006, 39, 1913. (b) Gendler, S.;
Groysman, S.; Goldschmidt, Z.; Shuster, M.; Kol, M. J. Polym. Sci., Part
A: Polym. Chem. 2006, 44, 1136. (c) Furuyama, R.; Mitani, M.; Mohri,
J.-I.; Mori, R.; Tanaka, H.; Fujita, T. Macromolecules 2005, 38, 1546. (d)
Tang, L.-M.; Hu, T.; Pan, L.; Li, Y.-S. J. Polym. Sci., Part A: Polym.
Chem. 2005, 43, 6323. (e) Jensen, T. R.; O’Donnell, J. J., III; Marks, T. J.
Organometallics 2004, 23, 740. (f) Jensen, T. R.; Yoon, S. C.; Dash, A.
K.; Luo, L.; Marks, T. J. J. Am. Chem. Soc. 2003, 125, 14482. (g) Chien,
J. C. W.; Yu, Z.; Marques, M. M.; Flores, J. C.; Rausch, M. D. J. Polym.
Sci., Part A: Polym. Chem. 1998, 36, 319. (h) Sernetz, F. G.; Mu¨lhaupt,
R. J. Polym. Sci., Part A: Polym. Chem. 1997, 35, 2549. (i) Patten, T. E.;
Novak, B. M. J. Am. Chem. Soc. 1996, 118, 1906.
(12) Polynuclear organo-group 4-mediated polymerization of functionalized and
sterically hindered monomers: (a) Li, H.; Marks, T. J. In ref 1a, p 15295.
(b) Li, H.; Li, L.; Schwartz, D. J.; Metz, M. V.; Marks, T. J.; Liable-
Sands, L.; Rheingold, A. L. J. Am. Chem. Soc. 2005, 127, 14756. (c) Guo,
N.; Li, L.; Marks, T. J. J. Am. Chem. Soc. 2004, 126, 6542. (d) Li, H.; Li,
L.; Marks, T. J.; Liable-Sands, L.; Rheingold, A. L. J. Am. Chem. Soc.
2003, 125, 10788. (e) Abramo, G. P.; Li, L.; Marks, T. J. J. Am. Chem.
Soc. 2002, 124, 13966.
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