Synthesis, characterizations and biocompatibility of novel biodegradable star block copolymers based on poly[(R)-3-hydroxybutyrate] and poly(ε-caprolactone)

Star block copolymers based on poly[(R)-3-hydroxybutyrate] (PHB) and poly(ε-caprolactone) (PCL), termed SPHBCL, were successfully synthesized with structural variation on arm numbers and lengths via coupling reactions and ring opening polymerizations. Arm numbers 3, 4 and 6 of SPHBCL were synthesized by using different multifunctional cores, such as trimethyol propane, pentaerythiritol and dipentaerthritol, respectively. Gel permeation chromatography (GPC) and 1H and 13C nuclear magnetic resonance were used to characterize the structure of SPHBCL. GPC failed to produce accurate molecular weights of the SPHBCL due to the discrepancy of star copolymer structures. The melting temperature of SPHBCL decreased with increasing degree of branching. Thermal decomposition temperature was revealed to be lower than that of linear block copolymer LPHBCL counterparts based on PHB and PCL. Films made from various SPHBCL copolymers had different porous or networking surface morphology, and all possessed improved biocompatibility in terms of less blood clotting and more osteoblast cell growth compared with their corresponding homopolymers PHB and PCL. Among them, it was found, however, that the 4-arm star block copolymer 4SPHBCL-25 showed unique surface properties, i.e. a regular nanoravine structure was observed by scanning electron microscopy and atomic force microscopy. This 4-arm star copolymer also showed the best biocompatibility.