Green Synthesis of Poly( ε ‐caprolactone) Using Environmentally Benign Organic Acid Catalysts: A Sustainable, Metal‐Free, and Solvent‐Free Route to Ring‐Opening Polymerization

Metal‐free polymerization is required to prevent contamination of polymeric materials by residual metal catalysts. In this study, the ring‐opening polymerization (ROP) of ε ‐caprolactone (CL) is investigated under bulk conditions at 100°C using benzyl alcohol (BnOH) as the initiator and seven environmentally benign organic acids (EBOAs) as activators (catalysts). In all cases, ROP yields poly( ε ‐caprolactone) (PCL) with narrow polydispersities ( Ð ). Kinetic analysis shows first‐order behavior, indicating a controlled process. The reaction rate constant ( k ) correlates with the acidity (p K a ) of EBOAs, with stronger acids (lower p K a ) exhibiting higher catalytic activity among those tested. Substituting BnOH with alternative alcohol initiators produces PCL bearing well‐defined heterotelechelic end groups, such as ally or polyethylene glycol moieties. Hydroxy‐terminated PCL enables further post‐polymerization, including chain extension and block polymerization (poly( ε ‐caprolactone)– b– poly( δ ‐valerolactone); PCL– b– PVL) with δ ‐valerolactone using EBOAs, while preserving narrow Ð . Moreover, the composition unit in PCL– b– PVL can be tuned by selecting EBOAs of different acidity, directly influencing catalytic activity. This controlled EBOA‐based system is useful for applications where metal contamination must be avoided, such as polymeric biomaterials.