Toxicity of Tin-Based Catalysts on Polymer-Degrading Bacteria

Biodegradable polymers are synthesized using a wide range of transition metal catalysts to control the molecular properties of the final polymer. While catalysts are traditionally chosen only for their efficacy in polymerization, they may also be toxic to microorganisms, impacting the measured biodegradation rate of the final polymer. In particular, catalyst toxicity could cause otherwise biodegradable materials to fail biodegradation testing, undervaluing the actual degradability of the polymer. Tin-based catalysts are widely used to synthesize biodegradable polymers through ring-opening polymerization of lactones due to their availability, cost, and effectiveness; however, their toxicity in environments relative to biodegradation is not well understood. Existing studies suggest toxic effects of tin in aquatic microorganisms with little focus towards the effect on bacteria important for biodegradation. In this study, we first quantify the toxicity of widely used tin-based catalysts in culture media on common soil bacteria with key roles in biodegradation. Using this information to assess the relevant concentration range, we then use clear-zone assays to measure the direct effects of these catalysts on biodegradation of solid state plastics and exemplify the extent of these effects using realistic concentrations of a variety of catalysts doped into commercial polymers. Combined, these results indicate the role that residual catalyst plays in affecting biodegradation results, and the high-throughput methods applied here provide a basis for catalyst screening to identify catalysts that best promote polymer biodegradation.