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 ≥ 11. On the other hand, in blends of d- and l-oligomers of lactic acid
crystallinity (stereocomplexation) was observed at a DP ≥ 7.
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