Effect of metal triflates on direct polycondensation of lactic acid

Various metal triflates were employed for direct polycondensation of lactic acid to afford poly(lactic acid). Screening of the metal triflates was conducted with the polycondensation of lactic acid at 160–180 °C for 16 h under reduced pressure (1.4 kPa) after a pre-dehydration process under an air atmosphere. Examined metal triflates are scandium, yttrium, ytterbium, lanthanum, hafnium, copper, and silver triflates. Polycondensation using scandium triflate afforded poly(lactic acid) with high molecular weights (M _n = 4.3 × 10⁴) in a good yields (62%) under the following conditions; catalyst amount = 0.05 mol%, pre-dehydration at 180 °C for 2 h, and polycondensation at 180 °C for 16 h. Active metal triflates coordinated with lactic acid strongly, confirmed by ¹H NMR and FT-IR spectroscopy.