Polypropylene (PP) is one of the most highly utilized engineering plastics, with production in the United States and Canada of over 8 million metric tons in 2003.1 The starting material for PP, propylene, can be transformed into polymers with a range of physical properties by adjusting the relative stereochemistry of the pendant methyl groups on the polymer backbone. For instance, when the

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methyl groups are placed in a stereorandom configuration, the resulting polymer, known as atactic polypropylene, is amorphous and lacks crystallinity. The vast majority of polypropylene sold is isotactic polypropylene (iPP), a resin valued for its strength, toughness, chemical and heat resistance, and processability. The highly stereoregular structure of iPP, in which the pendant methyl groups reside on the same side of the polymer chain, imparts a crystalline structure to the polymer and allows the material to have a high melting point (160-165 ◦C).2,3 A versatile plastic, iPP can be found in a wide range of applications from packaging to fibers to containers. Commercial, highly crystalline iPP is largely synthesized using traditional heterogeneous Ziegler-Natta catalysis, but the advent of homogeneous group 4 metallocenes has enabled polyolefin producers to access a wider range of polymer microstructures and properties. The mechanism of stereoselective polymerization by these catalysts is now well understood; the issues of stereocontrol and influence of the metallocene catalyst structure on the propylene polymerization behavior have been discussed in detail in earlier reviews.4-6 Excellent review articles are also available on the synthesis of chiral group 4 metallocenes.7,8 This chapter will introduce some fundamental aspects of metallocenecatalyzed propylene polymerization, discuss the details of how the C2-symmetry of the catalyst is exploited in the synthesis of iPP, and then survey the general ligand structures that are commonly encountered.