Ring-opening polymerization of ϵ-caprolactone by iodine charge-transfer complex

The ring-opening polymerization of e-caprolactone (e-CL) catalyzed by iodine (I 2 ) was studied. The formation of a charge-transfer complex (CTC) among triiodide, I - 3 , and e-CL was confirmed with ultraviolet-visible spectroscopy. The monomer e-CL was polymerized in bulk using I 2 as a catalyst to form the polyester having apparent weight-average molecular weights of 35,900 and 45,500 at polymerization temperatures of 25 and 70 °C, respectively. The reactivity of both, e-CL monomer and ∈-CL:I 2 CTC, was interpreted by means of the potential energy surfaces determined by semiempirical computations (MNDO-d). The results suggest that the formation of the ∈-CL:I 2 CTC leads to the ring opening of the e-CL structure with the lactone protonation and the formation of a highly polarized polymerization precursor (∈-CL) + . The band gaps approximated from an extrapolation of the oligomeric polycaprolactone (PCL) structures were computed. With semiempirical quantum chemical calculations, geometries and charge distributions of the protonated polymerization precursor (∈-CL) + were obtained. The calculated band gap (highest occupied molecular orbit/lowest unoccupied molecular orbit differences) agrees with the experiment. The analysis of the oligomeric PCL isosurfaces indicate the existence of a weakly lone pair character of the C=O and C-O bonds suggesting a e-CL ring-opening specificity.