C O M M U N I C A T I O N S
indicating that efficient silanolysis is both inter- and intramolecular
when the silyl group is held proximate to the Ti center either by
insertion into the growing polymer chain or by interaction of the
weakly basic silyl group to the Ti center (presumably via Scheme
1, steps iii and iv).
These results show 5-hexenylsilane to be a versatile comonomer
with the ability to both incorporate into a growing polyolefin chain
as well as to terminate growing polymer chains, thereby producing
branched, silyl-capped polyethylenes. This comonomer allows
coupled olefin enchainment and silanolytic chain transfer, thus
increasing the probability of branch formation.
Acknowledgment. Financial support by the NSF (Grant CHE-
04157407) is gratefully acknowledged. We thank Dr. A. Arzadon,
N. Guo, and J. Roberts for helpful discussions.
Figure 2. Relationship of polyethylene number average molecular weight
(GPC versus polyethylene) to (a) inverse C6H11SiH3 concentration at fixed
catalyst and ethylene concentrations, and (b) inverse C6H13SiH3 concentra-
tion at fixed catalyst and ethylene concentrations.
Supporting Information Available: Detailed experimental pro-
cedures, kinetic analysis, and polymer characterization data (Tables S1,
S2) are provided. This material is available free of charge via the
that 5-hexenylsilane readily undergoes insertion into the polymer
chain as well as effects intramolecular chain termination.
Previously, it was found that organotitanium-mediated ethylene
polymerization in the presence of alkyl- or arylsilanes fails to
produce silyl-capped polyethylenes, and that instead, vinyl-
terminated polyethylenes are produced, implicating â-hydride
elimination as the dominant chain transfer mechanism.4a Assuming
for any given reaction, constant [silane], [ethylene], and [catalyst]
and that rapid reinitiation occurs after chain transfer, where
silanolysis is the dominant chain transfer pathway, the number
average degree of polymerization Pn at ideal steady-state in [II]
should obey eq 1.10-12 Here kp and kSi are the rate constants for
chain propagation and (inter/intramolecular) silanolytic chain
transfer, respectively. Note from Figure 1 that 5-hexenylsilane is
an efficient chain transfer agent for organotitanium-mediated
ethylene polymerizations at 25 °C. With the polymerizations carried
References
(1) Effects of functional groups and branching on polyolefin properties: (a)
Chung, T. C. Prog. Polym. Sci. 2002, 27, 39. (b) Imuta, J.-I.; Kashiwa,
N.; Toda, Y. J. Am. Chem. Soc. 2002, 124, 1176. (c) Dong, J. Y.; Wang,
Z.; Hong, H.; Chung, T. C. Macromolecules 2002, 35, 9352. (d) Chung,
T. C. Polym. Mater. Sci. Eng. 2001, 84, 33. (e) Chum, P. S.; Kruper, W.
J.; Guest, M. J. AdV. Mater. 2000, 12, 1759.
(2) Recent reviews in olefin polymerization and copolymerization: (a) Gibson,
V. C.; Spitzmesser, S. K. Chem. ReV. 2003, 103, 283. (b) Pedeutour, J.-
N.; Radhakrishnan, K.; Cramail, H.; Deffieux, A. Macromol. Rapid
Commun. 2001, 22, 1095. (c) Chen, Y.-X.; Marks, T. J. Chem. ReV. 2000,
100, 1391. (d) Gladysz, J. A., Ed. Chem. ReV. 2000, 100 (special issue
on “Frontiers in Metal-Catalyzed Polymerization”).
(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) Koo, K.; Marks, T. J. J. Am. Chem. Soc. 1999,
121, 8791. (b) Koo, K.; Fu, P.-F.; Marks, T. J. Macromolecules 1999,
32, 981. (c) 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) Chung, T. C.; Dong, J. Y. J. Am.
Chem. Soc. 2001, 123, 4871.
Pn )
k
total [olefin]
∑
p
kSiinter[5-hexenylsilane] + kp5-hexenylsilane[5-hexenylsilane]
(1)
(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) (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.
out at constant catalyst and monomer concentration and a pseudo-
zero-order excess of 5-hexenylsilane, Figure 2a shows that eq 1 is
obeyed.11,12 Using this result and the data in Figure 1a yields
kp5-hexenylsilane/kSi ≈ 5 and kpethylene/kSi ≈ 180, arguing that chain
transfer predominantly occurs after 5-hexenylsilane enchainment.13
To assess any role for the silane CdC functionality in chain
transfer, control polymerizations were next carried out using
n-hexylsilane as chain transfer agent. 1H NMR integration of SiH2
versus vinyl resonances indicates predominantly vinyl-terminated
polyethylenes (Table S2), consistent with literature precedent.4a
Furthermore, the plot of Mn versus 1/[n-hexylsilane] evidences
nonideal chain transfer (Figure 2b) with the near-zero slope,
indicating that kSi/kp ≈ 0 (Figure 2b), and that silanolytic chain
transfer is no longer the dominant termination pathway. Silanolytic
chain transfer is then most likely competitive with â-hydride
elimination. That 5-hexenylsilane is a more efficient chain transfer
agent than n-hexylsilane (kp/kSitotal for n-hexylsilane is 150-fold that
of 5-hexenylsilane) indicates that the olefinic moiety is essential
in 5-hexenylsilane chain transfer. An attractive explanation is that
the chain transfer rates are enhanced by high local silane concentra-
total
total
(9) Organosilanes in organic synthesis: (a) ComprehensiVe Organic Func-
tional Group Transformations II; Katritzky, A. R., Taylor, R. J., Eds.;
Elsevier: Boston, 2005; Vols. 1, 2, 4, 5. (b) ComprehensiVe Organic
Functional Group Transformations; Katritzky, A. R., Meth-Cohn, O.,
Rees, C. W., Eds.; Pergamon Press: New York, 1995; Vols. 1-5. (c)
ComprehensiVe Organic Synthesis; Trost, B. M., Fleming, I., Eds.;
Pergamon Press: New York, 1991; Vols. 2, 6, 8. (d) Thomas, S. E.
Organic Synthesis: The Roles of Boron and Silicon; Oxford University
Press: New York, 1991. (e) Colvin, E. Silicon in Organic Synthesis;
Butterworth and Co. Ltd.: Boston, 1981.
(10) See Supporting Information for details.
(11) That the Mn intercept at infinite [5-hexenylsilane] is nonzero reflects the
imprecision of GPC data for such low polyethylene molecular weights.
(12) Olefin polymerization kinetics: (a) Tait, P. J. T.; Watkins, N. D.
ComprehensiVe Polymer Science; Allen, G., Bevington, J. C., Eds.;
Pergamon Press: Oxford, 1989; Vol. 4, pp 549-563. (b) Kissin, Y. V.
Isospecific Polymerization of Olefins; Springer-Verlag: New York, 1985;
pp 1-93.
(13) 13C NMR spectral assignments: Henschke, O.; Knorr, J.; Arnold, M. J.
Macromol. Sci., Chem. 1998, A35, 473.
tions in proximity to the electrophilic Ti center. Using eq 1 and
n-hexylsilane
the data in Figure 2 yields kSi5-hexenylsilane/kSi
g 150,
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