The ring-opening polymerization of epsilon-caprolactone (epsilon-CL) catalyzed by iodine (I-2) was studied. The formation of a charge-transfer complex (CTC) among triiodide, I-3(-), and epsilon-CL was confirmed with ultraviolet-visible spectroscopy. The monomer E-CL was polymerized in bulk using 1, 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 70degreesC, respectively. The reactivity of both, epsilon-CL monomer and epsilon-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 epsilon-CL:I-2 CTC leads to the ring opening of the epsilon-CL structure with the lactone protonation and the formation of a highly polarized polymerization precursor (epsilon-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 (epsilon-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 epsilon-CL ring-opening specificity.