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

To further verify the mechanism of forming multi-core structure during the direct melt copolycondensation of lactic acid (LA) with compounds containing multi-hydroxyl groups, the biodegradable material poly(lactic acid-co-sorbitol) [P(LA-co-SB)] was synthesized by using D,L-lactic acid (D,L-LA) and sorbitol (SB) as starting materials. For the molar feed ratio n(LA)/n(SB) of 120/1, optimal synthetic conditions were investigated. After prepolymerization at 140 °C for 8 h, melt copolymerization with the catalysis of SnO (0.5 wt %) at 160 °C for 6 h gave a polymer with the biggest intrinsic viscosity ([η]) 0.91 dL•g⁻¹. The copolymer P(LA-co-SB)s obtained 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). The results show that the weight-average molecular weight (M_w) doesn’t increase all the time along with the increasing of the molar feed ratio n(LA)/n(SB), but a M_w peak value exists, which validates again the special M_w peak phenomenon during the direct melt copolycondensation of LA with the monomers containing multifunctional groups (including polyols, e.g. glycerol). However, compared with the results of using glycerol as the core, the mechanism of forming the multi-core copolymer is somewhat different due to SB’s different structure. All the results show that the M_w peak value of the copolymers with multi-core structure is related to not only the number of hydroxyl groups in polyols, but also the reactivity of hydroxyl groups resulted from their type, and the steric hindrance of hydroxyl groups in the polyol.