Synthesis of Orientationally Isomeric Cyclic Stereoblock Polylactides with Head-to-Head and Head-to-Tail Linkages of the Enantiomeric Segments.

A pair of orientationally isomeric cyclic stereoblock polylactides (PLAs) possessing head-to-head (HH) and head-to-tail (HT) linkages between the poly(l-lactide) (PLLA) and poly(d-lactide) (PDLA) segments was synthesized through click chemistry and ring-closing metathesis (RCM) of three asymmetrically functionalized telechelic precursors. Thus, α-ethenyl-ω-azido-PLLA () was reacted with α-ethenyl-ω-ethynyl-PDLA () and α-ethynyl-ω-ethenyl-PDLA () via click chemistry to form ethenyl telechelic stereoblock PLAs with HH and HT orientations ( and ), respectively. The subsequent RCM produced cyclic stereoblock PLAs with the corresponding linking manners ( and ). The effect of the topology on the melting temperature of the series of isomeric linear and cyclic PLAs having the contrastive linking orientations was systematically investigated.