Synthesis and purification of poly(l-lactic acid) using a one step benign process

Abstract

In this study, supercritical carbon dioxide (scCO₂) was used as an alternative solvent for the synthesis and purification of poly(L-lactic acid) (PLLA) with a control on molecular weight. The synthesis of low molecular weight PLLA is desirable for applications such as drug delivery, resorbable implant applications and copolymerization of injectable polymer. Polymerizations were carried out in the presence of tin(II) 2-ethylhexanoate (Sn(Oct)₂) and diethylene glycol (DEG) as catalyst and initiator, respectively. The resultant polymers were characterized and analysed by GPC, ¹H NMR, ATR-FTIR, TGA and DSC. In particular, the effects of temperature, reaction time and pressure on polymer number average molecular weight (M_n), yield and polydispersity index (PDI) were investigated. Statistical analysis showed that temperature had the most significant effect on M_n, yield and PDI; this was followed by time and then pressure. An increase in temperature led to a significant increase in M_n, yield and PDI due to enhanced reactants solubility in CO₂. Time and pressure had considerable effect on M_n and yield but not PDI. The optimum conditions for synthesising low molecular weight PLLA was 80 °C, 160 bar and 17 hours. Similar conditions were also tested and found to be efficient for removing the impurities. The results of this study demonstrated the feasibility of using a one pot process for the synthesis and purification of PLLA. We achieved high yield, very low PDI and tuneable M_n ranging from 1200 to 13700 g mol⁻¹ within the range examined. The process allowed eliminating the use of toxic organic solvent, stabilizer and esterification promoting agent. The synthesis of low molecular weight PLLA is desirable for applications such as drug delivery, resorbable implant applications and copolymerization of injectable polymer.