A. Arbaoui and C. Redshaw (Polym. Chem., 2010, 1, 801) review catalysts for the synthesis of biodegradable polymers via ring opening metathesis polymerization. Summarize the need for biodegradable polymers and why new catalysts are required. From the details given identify which group of metals give the most active catalysts with which types of ligands. Illustrate with examples.