Thermal stability of polylactide with different end-groups depending on the catalyst used for the polymerization

Abstract

Thermal stability of polylactides (Mn∼4000) containing a different number of hydroxyl and carboxyl end groups synthesized by cationic ring-opening polymerization was studied under non-isothermal conditions. Polylactides (PLAs) were prepared in the presence of hydroxy acids or functional alcohols as initiators and protic acid as a catalyst and in some cases were subjected to the esterification with an acid anhydride in order to increase the number of carboxyl groups. Polymers purified by simple precipitation were characterized by 1H NMR and size exclusion chromatography (SEC) and subsequently analyzed by thermogravimetric analysis (TGA) which indicated that the degradation proceeded in a similar temperature range independently on the end-groups type. An unexpected observation was that the temperatures of the maximum rate of their decomposition were by 40÷78 °C higher than those determined for analogous PLA prepared (for comparison) in the presence of tin octoate. To exclude the possibility that this effect was due to the different structure of polymers prepared by different mechanisms the traces of tin octoate were added to PLA obtained by cationic polymerization and it was observed that this led to the decrease in temperature of maximum decomposition to the level observed for PLA prepared by coordination polymerization. Thus, we conclude that traces of protic acid used as a cationic catalyst (if present after applying a typical purification procedure) do not have an effect on thermal degradation in contrast to traces of metalorganic catalyst which, as shown earlier, promote thermal degradation.