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 1 H 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 °C. The glass transition temperature ( T g ) of the low molecular weight prepolymer was 47.8 °C. After chain extension, T g increased to 53.2 °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. Copyright © 2008 Society of Chemical Industry