Thermoresponsive Star Triblock Copolymers by Combination of ROP and ATRP: From Micelles to Hydrogels

Novel biocompatible, biodegradable, four-arm star, triblock copolymers containing a hydrophobic poly(epsilon-caprolactone) (PCL) segment, a hydrophilic poly(oligo(ethylene oxide) 475 methacrylate) (POEOMA475) segment and a thermoresponsive poly(di(ethylene oxide) methyl ether methacrylate) (PMEO(2)MA) segment were synthesized by a combination of controlled ring-opening polymerization (ROP) and atom transfer radical polymerization (ATRP). First, a four-arm PCL macroinitiator [(PCL-Br)(4)] for ATRP was synthesized by the ROP of epsilon-caprolactone (CL) catalyzed by stannous octoate in the presence of pentaerythritol as the tetrafunctional initiator followed by esterification with 2-bromoisobutyryl bromide. Then, sequential ATRP of oligo(ethylene oxide) methacrylate (OEOMA475, M-n = 475) and di(ethylene oxide) methyl ether methacrylate) (MEO(2)MA) were carried out using the (PCL-Br)(4) tetrafunctional macroinitiator, in different sequence, resulting in preparation of (PCL-b-POEOMA475-b-PMEO(2)MA)(4) and (PCL-b-PMEO(2)MA-b-POEOMA475)(4) star triblock copolymers. These amphiphilic copolymers can self-assemble into spherical micelles in aqueous solution at room temperature. The thermal responses of the polymeric micelles were investigated by dynamic light scattering and ultraviolet spectrometer. The properties of the two series of copolymers are quite different and depend on the sequence distribution of each block along the arms of the star. The (PCL-b-POEOMA475-b-PMEO(2)MA)(4) star copolymer, with the thermoresponsive PMEO(2)MA segment on the periphery, can undergo reversible sol-gel transitions between room temperature (22 degrees C) and human body temperature (37 degrees C). (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 49: 1942-1952, 2011