Influence of polymerization conditions on the hydrolytic degradation of poly( -lactide) polymerized in the presence of stannous octoate or zinc-metal

Laboratory- and pilot-scale racemic polylactides (PLA50) were synthesized in the presence of stannous octoate (SnOct2) or zinc-metal as initiators in the absence of alcohol. The resulting polymers were processed by compression molding or injection molding depending on the batch scale. The hydrolytic degradation of compression-molded samples selected to be comparable was investigated first in order to show the influence of the initiator system. Differences in water uptake were found between PLA50–Zn (zinc-metal initiation) and PLA50–Sn (SnOct2 initiation), PLA50–Zn being much more hydrophilic. PLA50–Sn exhibited a slower molecular weight decrease and delayed onsets of weight loss, release of acidity and stereocomplex formation, with respect to PLA–Zn. The concentration in residual tin in PLA50–Sn increased from 306 to 795 ppm during aging. In the case of PLA50–Zn the residual metal remains constant at ca. 40 ppm. In a second series of experiments, high molecular weight PLA50 different in characteristics and in initiator, synthesized under pilot-scale, were compared. The effects of the initiator on the degradation of the polymers well agreed with laboratory-scale findings, differences in hydrophobicity being enlarged by the up scaling. PLA50–Sn polymers appeared much more degradation resistant than PLA50–Zn ones. Contributions of the other characteristics (e.g. molecular weight, purity, stereoregularity, processing) were shown to be important as well.