Melt polycondensation of L-lactic acid to poly(L-lactic) with Sn(II) catalysts combined with various metal alkoxides

Melt polycondensation of (L)-lactic acid (LA) was examined in the presence of binary catalyst systems consisting of SnCl(2)(.)2H(2)O and metal alkoxides as co-catalysts. Among the co-catalysts examined, viz (A1((OPr)-Pr-i)(3),Ti((OPr)-Pr-i)(4),Y((OPr)-Pr-i)(3),Si(OEt)(4) and Ge(OEt)(4)), Ge(OEt)(4) was found to be the most effective in enhancing the catalytic activity of Sn(II). With an optimized composition of SnCl(2)(.)2H(2)O-Ge(OEt)(4), the molecular weight (M-n) of PLLA reached 40 000 Da in a short reaction time (<15h) at the optimum reaction conditions of 180degreesC and 10 Torr. This catalyst system was also superior to the conventional single metal ion catalysts such as Sn(II) in terms of racemization and discolouration of the resultant polymer. The metal alkoxides, added as co-catalysts, should work as oxo acids that can effectively control the catalytic activity of Sn(II) ion in the direct polycondensation of LA, in a manner similar to that of proton acids. (C) 2003 Society of Chemical Industry.