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 M-w/M-n 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 greater than or equal to 11. On the other hand, in blends of D- and L-oligomers of lactic acid crystallinity (stereocomplexation) was observed at a DP greater than or equal to 7.