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 T-m and Delta H-m 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 T-m and Delta H-m was also observed. Modification of the hydroxyl ends to produce COOH, -NH2 and -SH end groups reduced the stereocomplex T-m values to similar to 10-20 degrees C. The lower limit for the crystallization of the stereocomplexes was found at a DP of 5. (C) 2012 Elsevier Ltd. All rights reserved.