Web Release Date: September 1,
A Kinetic Study of Ethylene and 1-Hexene Homo- and Copolymerization Catalyzed by a Silica-Supported Cr(IV) Complex: Evidence for Propagation by a Migratory Insertion Mechanism
Contribution from the Centre for Catalysis Research and Innovation, Department of Chemistry, University of Ottawa, Ottawa, Ontario, Canada K1N 6N5
Received May 16, 2000
Abstract:
The surface organometallic fragment (
SiO)2Cr=CHCMe3 initiates the polymerization of ethylene
and 1-hexene at room temperature in the absence of organoaluminum or other activators. The kinetics at the
gas-solid interface were studied by in situ IR spectroscopy in a constant volume, variable pressure reactor. At
low pressures, the reaction is first order in P(C2H4), first order in the number of moles of chromium, and
linearly dependent on the Cr loading on silica. The second-order rate constant for polymerization is (177 ± 3)
s-1 (mol Cr)-1 at 21 
C, and is virtually unchanged upon subsequent additions of ethylene to the catalyst. The
kinetic isotope effect, k(C2H4)/k(C2D4), is a meagre 1.29. The temperature dependence of the second-order
rate constants yields 
H
obs = (30.2 ± 0.9) kJ/mol and Sobs = (-99 ± 3) J/(K·mol). These values are
consistent with a propagation mechanism of agostically assisted migratory insertion at an alkylchromium(IV)
active site. They are inconsistent with an alternating alkylidene/metallacycle mechanism requiring rate-determining C-H cleavage. The rate of polymerization is accelerated by a factor of 3.2 in the presence of 18
mol % H2. Homopolymerization of 1-hexene is only 4.7 times slower than that of ethylene, an effect attributed
to weaker preequilibrium binding of the substituted olefin which is partially compensated by faster migratory
insertion. Copolymerization of ethylene and 1-hexene is random. It results in slower incorporation of ethylene
but faster incorporation of 1-hexene relative to their respective homopolymerization rates.
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