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

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 (M_w) didn’t increase all the time, but a peak of M_w 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 M_w 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.