Synthesis and characterization of biodegradable lactic acid-based polymers by chain extension

BACKGROUND: Poly(lactic acid) (PLA), coming from renewable resources, can be used to solve environmental problems. However, PLA has to have a relatively high molecular weight in order to have acceptable mechanical properties as required in many applications. Chain-extension reaction is an effective method to raise the molecular weight of PLA. RESULTS: A high molecular weight biodegradable lactic acid polymer was successfully synthesized in two steps. First, the lactic acid monomer was oligomerized to low molecular weight hydroxyl-terminated prepolymer; the molecular weight was then increased by chain extension using 1,6-hexamethylene diisocyanate as the chain extender. The polymer was characterized using H-1 NMR analysis, gel permeation chromatography, differential scanning calorimetry and Fourier transform infrared spectroscopy. The results showed that the obtained polymer had a M-n of 27 500 g mol(-1) and a M-W of 116 900 g mol(-1) after 40 min of chain extension at 180 degrees C. The glass transition temperature (T-g) of the low molecular weight prepolymer was 47.8 degrees C. After chain extension, Tg increased to 53.2 degrees C. The mechanical and rheological properties of the obtained polymer were also investigated. CONCLUSION: The results suggest that high molecular weight PLA can be achieved by chain extension to meet conventional uses. (C) 2008 Society of Chemical Industry.