?-Caprolactone/ZnCl2 complex formation: Characterization and ring-opening polymerization mechanism

ε-Caprolactone (ε-CL) has been mixed with ZnCl2 at different mol ratios. The resulting complex was characterized through 1H and 13C NMR spectroscopy in bulk and in solutions, differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), and optical microscopy. Ring-opening polymerization of ε-caprolactone [M] using ZnCl2 as an initiator [I] at different monomer/initiator ratios has been successfully performed in xylene. The molecular weight of poly(ε-caprolactone) (PCL) as measured by gel permeation chromatografy (GPC) was found to depend linearly on the [M]/[I] ratio. Theoretical calculations were carried out to understand the geometry of the complex and the operating ring-opening mechanism. Both experimental and computational results and the presence of methylene–chloride end group, confirmed by NMR, are in agreement with a coordination–insertion mechanism for the ring-opening polymerization proposed in this article.