Synthesis of Elastomeric Poly(propylene) Using Unsymmetrical Zirconocene Catalysts:  Marked Reactivity Differences of “Rac”- and “Meso”-like Diastereomers

Synthesis of “meso”- and “racemic”-like diastereomers of Me2Si(3-MeInd)(Ind)ZrCl2 (5 and 6, respectively) was achieved through either metathetical reactions between the dianion of Me2(3-MeIndH)(IndH) (4) with ZrCl4 or via amine elimination reactions, followed by fractional crystallization. Propylene polymerizations using meso-5 in the presence of methyl aluminoxane under a variety of conditions leads to the formation of low molecular weight, semicrystalline, low tacticity, poly(propylene) (PP). The dominant chain transfer mechanism in this case is shown to involve β-H transfer to monomer. In contrast, rac-6 provides higher molecular weight, semicrystalline, elastomeric poly(propylene) (elPP) under a variety of conditions; chain transfer in this case involves, predominantly, β-H transfer to Zr. The properties of elPP produced using catalyst 6 show a gradual change from a lightly, cross-linked elastomer to a poorly crystalline thermoplastic, depending on both polymer molecular weight and crystallinity as revealed by differential scanning calorimetry and tensile testing. In particular, more crystalline material exhibits a higher initial modulus, yielding behavior and lower strain to break than less crystalline material of equivalent molecular weight. These findings further define polymer properties for the synthesis of flexible elastomers using this class of catalysts.