Ring-Opening Polymerization of ε-Caprolactone Catalyzed by Zn/Co Double Metal Cyanide Catalysts: The Vital Role of Coordinated Methanol

Ring-opening polymerization (ROP) of ε-caprolactone (ε-CL) provides an efficient route to synthesize the widely used biodegradable polymer poly(ε-caprolactone) (PCL). Compared to homogeneous catalysts, heterogeneous double metal cyanide (DMC) catalysts offer the advantages of easy separation and recyclability, thereby improving product purity for the polymer industry. In this work, Zn/Co DMC catalysts are synthesized from cobalt cyanic acid (H 3 [Co(CN) 6 ]) and zinc 2-ethylhexanoate (Zn(EH) 2 ) using methanol as a solvent. The structure and composition of the prepared DMC catalyst are determined with comprehensive characterizations (e.g., ICP, elemental analysis, FTIR, TGA, XRD, and XPS). Kinetic studies of ROP of ε-CL catalyzed by the prepared Zn/Co DMC catalysts with and without an external initiator are systemically investigated, and the corresponding kinetic equations are developed as well. Results show that coordinated methanol exclusively initiates polymerization without an external initiator. Adding an external benzyl alcohol initiator or increasing catalyst loading accelerates polymerization but reduces the average molar mass of the resulting polymers. Finally, by integrating structural features with polymerization kinetics, a reaction mechanism for DMC-catalyzed ε-CL ROP is proposed. This mechanism delineates the functional role of each component, establishing a theoretical framework for advancing DMC catalyst applications.