Thermal stability of poly(L-lactide) prepared by polymerization of L, L-dilactide with Sn(II)-based initiators

Abstract

α-Ester-ω-hydroxy terminated poly(L-lactide) [C 4H9OC(O)-PLA-OH] was prepared by the ring-opening polymerization of L,L-dilactide using tin(II) 2-ethylhexanoate [tin octoate, Sn(Oct)2]/BuOH system or tin(II) butoxide as initiators in a wide range of molecular weights (M̄n): from 103 up to 7 · 105. Thermal degradation behavior of C4H 9OC(O)-PLA-OH was then compared with that of its esterified counterpart [C4H9OC(O)-PLA-OC(O)CH(C2H 5)C4H9]. Thermogravimetric measurements versus M̄n revealed, as a result of M̄n → ∞ extrapolation, the upper limit of thermal resistance of poly(L-lactide) (PLA). The onset temperature of the thermal degradation (OT) and the temperature of maximum degradation rate (MDT) of PLA are equal to 337 and 376°C, respectively. Despite the deteriorating effect on the PLA molecular weight caused by in situ esterification of the hydroxyl chain ends with Sn(Oct) 2, largely unreacted in polymerization, certain improvement of PLA thermal stability due to this side reaction was also observed.