2-Arylindene metallocenes: conformationally dynamic catalysts to control the structure and properties of polypropylenes.

Polymerization of propylene with catalysts derived from unbridged 2-arylindene metallocenes leads to elastomeric, low-tacticity polypropylenes. The polymerization behavior of these conformationally dynamic catalyst systems is interpreted in terms of a multistate mechanism where the catalyst enchains the monomer from both stereoselective and nonstereoselective conformations during the lifetime of a single polymer chain. The structure and properties of the polypropylenes depend sensitively on the metallocene and the polymerization conditions (temperature, pressure, monomer concentration); samples of low to intermediate isotacticity ([mmmm] = 20-40%) are elastomeric, with low densities and broad melting ranges that extend to 150 degrees C. Samples of higher tacticity ([mmmm] = 40-70%) behave as softened thermoplastics.