Silk sericin as a bio-initiator for grafting from synthesis of polylactide via ring-opening polymerization

Abstract

In this study, a new graft copolymer (SS-g-PLA) was constructed by combining sericin and polylactide (PLA) via ring-opening polymerization (ROP) having Sn(Oct)2 as a catalyst. In this synthetic strategy based on the grafting from concept, sericin molecule acts as a bio-initiator where its hydroxyl groups existing in serine units. The effects of polymerization parameters, including reaction temperature, the concentration of catalyst and sericin, on the copolymer structure and properties were investigated. XPS spectra of SS-g-PLA copolymer showed C1S and N1S signals clearly and differed form that of the neat PLA. 2D HMBC NMR spectra showed correlation assigned to the carbon atom of PLA polymer chains and hydrogen atom in hydroxyl group of serine. Molecular weight of the graft copolymer by GPC was in range of 4000–5600 g mol−1. From soxhlet extraction, the results showed that PLA content in the graft copolymer molecules increased with increasing sericin content. The weight ratio of PLA to sericin in the SS-g-PLA5, SS-g-PLA10 and SS-g-PLA15 graft copolymers were 0.6:1, 1.4:1 and 2.1:1, respectively. By nitrogen analyzer,%protein increased significantly with increasing sericin content. Thermal analysis showed that the graft copolymer presented a single glass-transition temperature (Tg) belonging to PLA. This Tg decreased with increasing sericin content. The thermal degradation temperature (Td) of SS-g-PLA copolymer decreased to about 60 °C. Furthermore, SS-g-PLA is expected to utilize in biomedical applications.