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 (2a) was reacted with α-ethenyl-ω-ethynyl-PDLA (2b) and α-ethynyl-ω-ethenyl-PDLA (2c) via click chemistry to form ethenyl telechelic stereoblock PLAs with HH and HT orientations (3a and 3b), respectively. The subsequent RCM produced cyclic stereoblock PLAs with the corresponding linking manners (4a and 4b). 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.