Kinetics of propylene polymerization using bis(2-phenylindenyl)zirconium dichloride/methylaluminoxane

The kinetics of propylene polymerization in toluene solution by bis(2-pheny lindenyl)zirconium dichloride/methylaluminoxane at 20 degreesC were investi gated. As the structure and properties of elastomeric polypropylenes produc ed by these catalysts depend sensitively on the reaction conditions, a deta iled study of the kinetics was carried out to evaluate the influence of the se parameters on the polymerization behavior. Studies of the solubilities a nd mass-transfer rates reveal that dissolved atactic polypropylene has litt le effect on propylene solubility but influences the mass-transfer rate of propylene into solution. The rates of propylene polymerization reach a maxi mum after 10-20 min and then decrease. The decrease in rate over time is fa ster 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 P-hydride elimination to be the primary chain-transfer mechanism. Narrow m olecular weight distributions (M-w/M-n = 2.0-2.6) were obtained. The increa se of the isotactic dyads and pentads ([m] and [mmmm]) with increasing mono mer concentration reveals an additional kinetic event which competes with t he stereodifferentiating olefin insertion step. Modeling studies are more c onsistent with a mechanism involving interconversion of the catalyst betwee n isospecific and aspecific states than a mechanism involving epimerization of the stereogenic centers of the growing polymer chain.