Synthesis of biodegradable material poly(lactic acid-co-glycerol) via direct melt polycondensation and its reaction mechanism

Abstract

To further verify the forming mechanism of multi-core structure during the direct melt copolycondensation of lactic acid (LA) with the compounds containing multifunctional groups, the biodegradable material poly(lactic acid-co-glycerol) [P(LA-co-GL)] was synthesized as designed using L-lactic acid (L-LA) and glycerol (GL) as the starting materials. For the molar feed ratio n(LA)/n(GL) of 60/1, the optimal synthetic conditions were discussed. Using 0.3 wt% stannous oxide (SnO) as the catalyst, after the prepolymerization was carried out at 140 °C for 8 h, the melt copolymerization for 8 h at 160 °C gave the polymer with the biggest intrinsic viscosity ([η]) 0.76 dL•g−1. The copolymers P(LA-co-GL)s at different molar feed ratios were characterized by Fourier transform infrared spectroscopy (FT-IR), proton nuclear magnetic resonance (1H-NMR), gel permeation chromatography (GPC), differential scanning calorimetry (DSC) and X-ray diffraction (XRD). Increasing the molar feed ratio n(LA)/n(GL), the weight-average molecular weight (Mw) didn’t increase all the time, but a peak of Mw was formed, which indeed validated the above special phenomenon during the direct melt copolycondensation of LA with the monomers containing multifunctional groups. However, the forming mechanism of multi-core copolymer was different when multihydroxyl alcohol (e.g. GL) was used as the monomer containing multifunctional groups. Because the multi-core structure was linked by the ether bonds with less reversibility in the reaction, the biggest Mw of copolymers was relatively lower. For GL with three terminal hydroxyls as the core, only when n(LA)/n(GL) was more than 100/1, the star-shaped polylactic acid (SPLA) containing one core could be obtained.