Bis(2-arylindenyl)zirconium dichlorides activated by methylaluminoxane produce elastomeric polypropylene with a stereoblock microstructure. In an effort to experimentally explore the importance of the 2-arylindene ligand in producing these materials, a series of catalysts was investigated that differed significantly from the parent ligand architecture. In particular, we sought to examine the influence of ligand aryl stacking on the polymerization behavior of these metallocenes. We discovered that the bis(2-arylindenyl) framework is important for the production of elastomeric polypropylene by this type of unbridged complex. Small changes in the fundamental catalyst structure upset the delicate balance between ligand rotation and monomer insertion resulting in the production of amorphous polymers.

Bis(2-arylindenyl)zirconium dichlorides activated by methylaluminoxane (MAO) produce elastomeric polypropylene. The elastic properties of this material are proposed to arise from a stereoblock microstructure. The silyl bridged catalysts rac- and meso-dimethylsilyl(bis(2-phenylindenyl))zirconium dichloride were prepared and used to model the polymerization behavior of the unbridged catalysts. The meso isomer is approximately twice as productive as the rac isomer in propylene polymerization. Additionally, blends of isotactic and atactic polypropylene were prepared by using varying amounts of the rac and meso isomers until the IR and NMR characteristics matched polypropylenes produced from bis(2-phenylindenyl)zirconium dichloride. These studies suggest (1) that the rac isomer is favored in the isomerization equilibrium for the unbridged catalysts and (2) that the larger fraction of atactic polymer in the elastomeric polypropylene is a consequence of the higher rate of polymerization for the meso isomer.

Trialkylaluminum reagents such as triisobutylaluminum (TIBA) and trimethylaluminum (TMA) influence the propylene polymerization behavior of methylaluminoxane (MAO)-activated dimethylsilyl[bis(2-phenylindenyl)]zirconium dichloride (1). TIBA selectively decreases the productivity of the meso diastereomer of 1 (1-meso). This is manifested in an increase in the isotacticity of polypropylene blends prepared from mixtures of 1-rac and 1-meso compared to polymer blends prepared in the absence of TIBA. In contrast, added trimethylaluminum decreases the productivity of both 1-rac and 1-meso. Similar trends in reactivity with AlR3 are observed for unbridged bis(2-phenylindenyl)zirconium dichloride (2) compared to a mixture of the bridged catalysts containing 57% 1-rac. Mechanical testing indicates that polypropylene from 2 has superior elastomeric properties to reactor blends prepared from mixtures of 1-rac and 1-meso in the presence and absence of trialkylaluminum reagents. Reactor blends prepared without AlR3 exhibit poorer recovery at 100% and 300% strain than that of the elastomeric polypropylene from 2. The reactor blends prepared in the presence of TIBA exhibit pronounced yielding and necking during testing, characteristic of a softened plastic.