Homogeneous metallocene catalysts, e.g., derived from or related to zirconocene dichloride introduced a level of microstructural control that was unavailable with heterogeneous systems.  Metallocene catalysts are homogeneous single-site systems, implying that a uniform catalyst is present in the solution.  In contrast, commercially important Ziegler-Natta heterogeneous catalysts contain a distribution of catalytic sites.  The catalytic properties of single-site catalysts can be controlled by modification of the ligand.  Initially ligand modifications focused on various cyclopentadienyl derivatives, but great diversity was uncovered through high throughput screening.  These post-metallocene catalysts employ a range of chelating ligands, often including pyridine and amido (RN).  These ligands are available in great diversity with respect to their steric and electronic properties.  Such postmetallocene catalysts enabled the introduction of Chain shuttling polymerization.