Monodisperse Enantiomeric Lactic Acid Oligomers: Preparation, Characterization, and Stereocomplex Formation

d- and l-lactic acid oligomers were synthesized by polymerization of d- or l-lactide using 2-(2-methoxyethoxy)ethanol (MEE) and stannous octoate as initiator and catalyst, respectively. The average degree of polymerization (DPav) of the oligomers could be tailored by the monomer/initiator ratio. HPLC and GPC analysis showed that the oligomers had a Mw/Mn ratio of around 1.5. Mass spectroscopic analysis revealed that products contained besides MEE−(lactate)n=2,4,6,etc. also MEE−(lactate)n=1,3,5,etc.. The latter products are most likely formed due to transesterification reactions. Monodisperse lactic acid oligomers (DP 1−16) were obtained from the polydisperse oligomers by preparative HPLC and characterized by NMR and mass spectrometry. DSC analysis showed that crystallinity was present in d- or l-oligomers with DP ≥ 11. On the other hand, in blends of d- and l-oligomers of lactic acid crystallinity (stereocomplexation) was observed at a DP ≥ 7.