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.