Synthesis and thermal properties of hetero-bifunctional PLA oligomers and their stereocomplexes

Hetero-telechelic, low-molecular-weight polylactides (PLAs) were prepared by the zinc-catalyzed ring-opening polymerization of l-lactide or d-lactide using functional initiators and subsequent reaction with termination reagents, yielding -OH, -COOH, -NH2 and -SH as functional chain ends. Structural characterization was performed by molecular weight analysis, NMR spectroscopy and MALDI-TOF mass spectrometry. The thermal behavior of the species was investigated by DSC, which revealed that the modification of the hydroxyl terminus lowered the number of lactic acid units within a PLA chain that can effectively participate in crystallization. Both the Tm and ΔHm values for these polymers were lower compared to those of PLAs of comparable chain length with no modification of the hydroxyl end-group. In stereocomplexes prepared from equimolar amounts of the hetero-telechelic PLLAs and PDLAs, this suppressive effect on Tm and ΔHm was also observed. Modification of the hydroxyl ends to produce -COOH, -NH2 and -SH end groups reduced the stereocomplex Tm values to ∼10-20 °C. The lower limit for the crystallization of the stereocomplexes was found at a DP of 5.