They act as Lewis acids and possess high catalytic activity. However, tin-based compounds and organotin compounds, such as DBTDL in particular, exhibit cytotoxicity, are difficult to remove from polymers, and have adverse environmental effects. Therefore, their usage in medical applications is limited and there is growing interest in "greener" organotin-free reaction pathways. Towards this aim, alternative catalytic systems are being explored that replace tin with other metals, such as zinc, titanium, zirconium, manganese, etc. Metal catalysts based on bismuth and zinc are of particular interest, due to their role in the human metabolism and use in pharmaceuticals, cosmetics, etc. Recently, they were tested in the synthesis of biodegradable polyesters, such as poly(ε-caprolactone) or poly(lactide). Overall, the activity of zinc and bismuth catalysts was similar, but lower than that of the organotin catalyst DBTDL.