The kinetics of propylene polymerization in toluene solution by bis(2-phenylindenyl)zirconium
dichloride/methylaluminoxane at 20 °C were investigated. As the structure and properties of elastomeric
polypropylenes produced by these catalysts depend sensitively on the reaction conditions, a detailed study of
the kinetics was carried out to evaluate the influence of these parameters on the polymerization behavior.
Studies of the solubilities and mass-transfer rates reveal that dissolved atactic polypropylene has little effect
on propylene solubility but influences the mass-transfer rate of propylene into solution. The rates of propylene
polymerization reach a maximum after 10−20 min and then decrease. The decrease in rate over time is faster
at higher monomer concentrations. Catalyst activity was negligible at [Al]/[Zr] = 1000 but constant from
[Al]/[Zr] = 2500 to [Al]/[Zr] = 10 000. Analysis of molecular weights as a function of monomer concentration
reveal β-hydride elimination to be the primary chain-transfer mechanism. Narrow molecular weight distributions
(M
w/M
n = 2.0−2.6) were obtained. The increase of the isotactic dyads and pentads ([m] and [mmmm]) with
increasing monomer concentration reveals an additional kinetic event which competes with the stereodifferentiating olefin insertion step. Modeling studies are more consistent with a mechanism involving interconversion
of the catalyst between isospecific and aspecific states than a mechanism involving epimerization of the
stereogenic centers of the growing polymer chain.
A new synthesis of unbridged mixed ring zirconocenes was developed and a series of mixed
ligand zirconocenes with substituted 2-arylindenyl and 1-methyl-2-arylindenyl ligands have
been prepared. When activated with methylaluminoxane, the mixed ligand complexes
catalyze the polymerization of propylene to give elastomeric polypropylene. The propylene
polymerization behavior of the mixed ligand catalysts was compared to that of their bis(indenyl) analogues to determine the relative contribution of each ligand to the activity and
stereospecificity of the catalyst. The effects of 2-arylindenyl and 1-methyl-2-arylindenyl
ligands on the stereospecificity of the catalysts are essentially additive; the stereospecificity
of the mixed ring complex is intermediate to that of the bis(2-arylindene) analogues. However,
the effect of 1-methyl substitution on catalyst productivity is not additive; the productivities
exhibited by bis(2-arylindenyl)ZrCl2/MAO and (1-methyl-2-arylindenyl)(2-arylindenyl)ZrCl2/MAO derived catalysts are similar and substantially higher than those of the bis(1-methyl-2-phenylindenyl)ZrCl2/MAO catalysts.
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