Precision Synthesis of Polylactide-Based Thermoresponsive Block Copolymers via Successive Living Cationic Polymerization of Vinyl Ether and Ring-Opening Polymerization of Lactide

摘要

A new class of polylactide (PLA)-based block copolymers with thermoresponsive poly(vinyl ether) [poly(VE)] were precisely synthesized via successive living cationic polymerization of VE and ring-opening polymerization of lactide (LA). The synthetic route starts with precise end-functionalization of poly(VE) by living cationic polymerization to produce a macroinitiator having a hydroxy group at the α- and/or ω-end for ring-opening polymerization of LA. End-functionalized polymers of 2-methoxyethyl VE (MOVE) with highly controlled structures were obtained by either an initiation or a termination method under optimized conditions. Subsequent ring-opening polymerization of LA from the hydroxy group of the macroinitiator yielded block copolymers with well-defined structures. The obtained block copolymers were demonstrated to exhibit a thermoresponsive solubility transition in water; this transition was induced by the thermosensitivity of the poly(MOVE) segments. The transition between micelle-like aggregates and phase separation was reversible with a shorter PLA chain, whereas irreversible precipitation was observed with a longer PLA segment. The permanent precipitation upon heating is most likely attributable to crystalline formation of the longer PLA segments. The solution and bulk properties of the block copolymers with enantiomeric PLA were also governed by stereocomplex formation between poly(l-lactide) (PLLA) and poly(d-lactide) (PDLA) segments.