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

Abstract

The ring-opening polymerization of ε-caprolactone (ε-CL) catalyzed by iodine (I2) was studied. The formation of a charge-transfer complex (CTC) among triiodide, I3-, and ε-CL was confirmed with ultraviolet-visible spectroscopy. The monomer ε-CL was polymerized in bulk using I2 as a catalyst to form the polyester having apparent weight-average molecular weights of 85,900 and 45,500 at polymerization temperatures of 25 and 70 °C, respectively. The reactivity of both, ε-CL monomer and ε-CL:I2 CTC, was interpreted by means of the potential energy surfaces determined by semiempirical computations (MNDO-d). The results suggest that the formation of the ε-CL:I2 CTC leads to the ring opening of the ε-CL structure with the lactone protonation and the formation of a highly polarized polymerization precursor (ε-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 (ε-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 ε-CL ring-opening specificity. © 2002 Wiley Periodicals, Inc. J. Polym. Sci. Part B: Polym. Phys.