Toward all stereocomplex-type polylactide with outstanding melt stability and crystallizability via solid-state transesterification between enantiomeric poly(l-lactide) and poly(d-lactide)

Abstract

Stereoblock copolymerization is regarded an appealing strategy to substantially enhance the melt stability (i.e., the survival ability of the intermolecular collaboration between poly(l-lactide) (PLLA) and poly(d-lactide) (PDLA) upon melting) of high-molecular-weight (high-MW) stereocomplex-type polylactide (SC-PLA), however it usually suffers from complicated synthetic procedures and even impaired sustainability. Herein, solid-state transesterification (SST) has been devised as a facile and robust route to prepare unique multi-stereoblock PLA copolymers with outstanding melt stability and unexpectedly strong crystallizability from commercial available linear high-MW PLLA/PDLA (50/50) blends. The SST was performed through low-temperature (i.e., 180 °C) melt-blending of PLLA and PDLA in the presence of trace amounts (0.03–0.035 wt%) of catalysts, where the enantiomeric PLA chains could rapidly co-crystallize into solid-state SC crystallites upon melting and subsequently the hetero-chain transesterification reaction occurs exclusively in the mobile amorphous phase of the pre-formed SC crystallites. As a result, multiblock-like PLLA-b-PDLLA-b-PDLA copolymers with long crystallizable PLLA and PDLA blocks are in-situ generated. The results show that high-content SC crystallites can be exclusively formed in the melt-crystallization of the copolymers even at high cooling rates (e.g., 40 °C/min) and low temperatures (e.g., 140 °C). More interestingly, the presence of the non-crystallizable PDLLA blocks not only does not disturb the SC crystallizability of the enantiomeric PLA blocks, as evidenced by the same high crystallinity and melting temperature as those of starting PLLA/PDLA blends, but also increase the crystallization rate by stabilizing the clusters of ordered PLLA/PDLA segments in the copolymer melt. Overall, these findings could open a new pathway to develop melt-processible all SC-PLA for engineering applications.