Synthesis and characterization of hyperbranched poly(ε-caprolactone)s having different lengths of homologous backbone segments

Hyperbranched poly(epsilon-caprolactone)s (HPCLs) were synthesized by moisture-sensitive catalyst-free polycondensation of AB(2) macromonomers, 2,2-bis[omega-hydroxy oligo(epsilon-caprolactone)methyl]propionic acids. The HPCLs were designed to incorporate different lengths of linear oligomeric segments consisting of 5, 10, and 20 epsilon-caprolactone monomer units on the branched backbone chains, accordingly named as HPCL-5, -10, and -20, respectively. End-group analyses were performed on H-1 NMR spectra of the three HPCLs, which provided information about the average number of C-caprolactone units incorporated in the AB(2) macromonomers and thus the average number of AB(2) macromonomer units incorporated in the resulting hyperbranched polymers. Consequently, the absolute values of molecular weights for the HPCLs were calculated. Size exclusion chromatography equipped with multiangle laser light scattering detector (SEC-MALLS) was employed to measure absolute molecular weights and molecular weight distributions of HPCLs. Then, the results were compared with those obtained from H-1 NMR end-group analyses, indicating that there was a good agreement between them. From small-angle X-ray scattering (SAXS), the radii of gyration of the HPCLs and their linear counterpart, poly(epsilon-caprolactone) (LPCL), were determined from the initial slope of the reciprocal of scattered intensity, 1/I(q), vs the square of scattering vector, q(2), Curves, fit by the Zimm scattering function. The ratio of mean-square radius of gyration of each HPCL to that of LPCL, termed the branching ratio, resulted in the relative degree of branching for individual HPCLs. As the length of linear oligo(c-caprolactone) segments decreased, the more highly branched polymers were obtained, i.e., in the order of HPCL-5, -10, and -20 from higher to lower degree of branching. The melting temperatures of HPCLs were lower than that of LPCL and decreased with increase in the degree of branching. In contrast, the thermal decomposition temperatures of HPCLs were shown to be higher than that of LPCL, becoming higher as the degree of branching increased.