Melt/solid-state polytransesterification supported by an inert gas flow – an alternative route for the synthesis of high molar mass poly(l-lactic acid)

Abstract

When the conventional method of melt polycondensation of lactic acid is carried out a product of low or medium molar mass (MM) unsuitable for commercial application may be obtained. In contrast, the ring opening polymerization of lactide (ROP), the commercially adopted method for the synthesis of poly(L-lactic acid) (PLLA), leads to a high molar mass (HMM) product. However, this polymerization method is also not free from drawbacks. The complicated purification process for the monomer (lactide) makes the cost of synthesizing PLLA high. Currently, a lot of effort is being put into developing an efficient and inexpensive route for the synthesis of PLLA with high MM. In our studies, PLLA of molar mass exceeding 100 000 g mol⁻¹ was obtained by melt/solid-state polytransesterification (SSPT) supported by an inert gas flow. Ethyl-, isopropyl-, and acetyl-terminated PLLA prepolymers were used. To the best of our knowledge, and in accordance with the applicable information there are no previous reports on the use of solid-state polytransesterification for the synthesis of PLLA. The obtained results (polymers with improved thermal stability T_d,max = 316 °C and M_w exceeding 125 000 g mol⁻¹) indicated that polytransesterification in the solid state can be envisaged to be a promising alternative for PLLA synthesis for both the conventional melt polycondensation as well as for the ROP of lactide. It is worth stating that no organic solvents were used during the synthesis of HMM PLLA which makes the described method environmentally friendly.