Tailoring polymer properties such as stiffness, strength, or processing requires control of the molecular architecture of the polymers. For the polymerization of olefins like propylene, the development of metallocene catalysts gives access to new polymer microstructures. During the last decade, major advances have been made in metallocene catalysis for polypropylenes, providing higher levels of control over composition, molecular mass distributions, and stereoregularity. This leads to a large variety of polypropylenes with varying types and degrees of stereoregularity,1-4 from amorphous or low-crystallinity types to highly stereoregular ones such as isotactic and syndiotactic polypropylene.4 Owing to the regular arrangement of methyl groups, these stereoregular polypropylenes are semicrystalline materials with melting points above 180 ◦C exhibiting typical thermoplastic deformation behavior (Thermoplast, Figure 9.1), but isotactic and syndiotactic polypropylene differ in elastic modulus and impact strength.5 In atactic polypropylenes, the statistically random arrangement of the methyl groups prevents crystallization. Therefore, atactic polypropylenes are soft, amorphous polymers ranging from oils to soft, waxy materials (Wax, Figure 9.1).6