Influence of tin(II), aluminum(III) and titanium(IV) catalysts on the transesterification of poly(L-lactic acid)

Abstract

The transesterification of poly(L-lactic acid) (PLA) with 0.05 M and 0.10 M of tin(II) octoate (Sn(Oct)2), aluminum(III) tri-sec-butoxide (Al(OsBu)3) and titanium(IV) n-butoxide (Ti(OnBu)4) catalysts in CHCl3 was successfully investigated by gel permeation chromatography (GPC) and proton-nuclear magnetic resonance spectroscopy (1H-NMR) techniques. All catalysts were completely and rapidly dissolved in PLA solution. When intramolecular transesterification reaction occurred, the physical appearances of PLA were changed from longer fiber to shorter fiber or power due to the decreasing of molecular weight and chain scission. The transesterification reaction of PLA depended on catalyst concentration, reaction time and catalyst type. From kinetics study, the transesterification of PLA with all catalysts was the first order reaction. The molecular weight of PLA decreased faster at higher catalyst concentration. At identical catalyst concentration, the molecular weight of PLA obtained from transesterification time of 1 d was higher than 2, 3 and 4 d, respectively. The molecular weight of PLA in the presence of Sn(Oct)2 dramatically decreased to 7.33 × 103 g/mol at 4 d when compared with PLA in the absence of Sn(Oct)2 (6.78 × 104 g/mol). From GPC technique, the apparent rate constants of transesterification (kapp) of PLA with 0.10 M of Sn(Oct)2, Ti(OnBu)4 and Al(OsBu)3 were 0.283, 0.085 and 0.073 d−1, respectively. From 1H-NMR technique, the kapp values of PLA with 0.10 M of Sn(Oct)2, Ti(OnBu)4 and Al(OsBu)3 were 0.297, 0.192 and 0.151 d−1, respectively. The lowest apparent half-life (t1/2,app) obtained from the 1H-NMR technique for PLA transesterification with Sn(Oct)2, Ti(OnBu)4 and Al(OsBu)3 was 2.333, 3.610 and 4.589 d, respectively. The transesterification activity of Sn(II), Al(III) and Ti(IV) catalysts for PLA was determined in the following order: Sn(Oct)2 > Ti(OnBu)4 > Al(OsBu)3. From kinetics and mechanistic studies, the transesterification of PLA with these Sn(II), Al(III) and Ti(IV) catalysts was proposed through the intramolecular transesterification. Graphical abstract: [Figure not available: see fulltext.]