Toxic Effect of Tin-Based Catalysts on Polymer Biodegradation

As the global plastic waste crisis intensifies, there is a growing need for the development of degradable materials to prevent the widespread environmental accumulation of plastics. As researchers have explored different chemical structures to find polymers with both favorable performance and biodegradability, little attention has been paid to residual polymerization catalysts and their effect on biodegradation. There has been some concern that residual transition metal catalysts could have a toxic effect on degrading microorganisms, especially in testing laboratory-synthesized polymers where higher catalyst loadings are used. This work explores the toxicity of three tin-based compounds widely used in the synthesis of biodegradable polyesters (tin(II)-acetate, tin(II)-octoate, and tin(II)-oxide) to common degrading species. In liquid cultures, catalyst toxicity can be observed to be as low as 0.025 mg/mL, while solid agar plates showed toxicity as low as 0.2 mg/mL, except for tin(II)-oxide, where no toxicity was observed. Using clear-zone assays, polymer biodegradation and species growth were not observed for residual catalyst concentrations above ∼1–2 mol %, demonstrating the enhanced toxic effect of these catalysts during biodegradation testing at industrially relevant concentrations. In all cases, toxicity was not observed equivocally; notably, Paucimonas lemoignei, a widely used polymer degrader, was strongly impacted by the presence of these catalysts.