Elastomeric Polypropylene from Unbridged 2-Arylindenyl Zirconocenes: Modeling Polymerization Behavior Using ansa-Metallocene Analogues

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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.