Core–shell microgels having zwitterionic hydrogel core and temperature-responsive shell prepared via inverse miniemulsion RAFT polymerization

Abstract

Stimuli-responsive core–shell microgels are of significant interest because of their fascinating applications due to the different swelling/shrinkage properties of their core and shell networks. Because such stimuli-responsive core–shell microgels are conventionally prepared by precipitation polymerization, hydrophilic and biological molecules are difficult to incorporate into stimuli-responsive core–shell microgels. We have focused on the preparation of stimuli-responsive core–shell microgels with zwitterionic hydrogel core by inverse miniemulsion RAFT polymerization method because it enables the facile incorporation of hydrophilic and biological molecules maintaining their functions. This study describes the preparation of core–shell microgels composed of zwitterionic poly(methacryloyloxyethyl phosphorylcholine) (PMPC) hydrogel core and temperature-responsive poly[oligo(ethylene glycol)methacrylate-co-2-(2′-methoxyethoxy)ethyl methacrylate] (P(OEGMA-co-MEO₂MA)) shell. A water-soluble block copolymer emulsifier composed of a hydrophilic/lipophilic P(OEGMA-co-MEO₂MA) block and a hydrophilic PMPC block was synthesized via reversible addition fragmentation chain transfer (RAFT) polymerization. The water-in-oil (W/O) emulsions were successfully formed in a water–chloroform two-phase system in the presence of the resulting P(OEGMA-co-MEO₂MA)-b-PMPC, because it stabilized the interface between water and chloroform by the distribution of PMPC and P(OEGMA-co-MEO₂MA) blocks in the water and chloroform phases, respectively. The inverse miniemulsion RAFT copolymerization of MPC and N,N′-methylenebisacrylamide proceeded from the P(OEGMA-co-MEO₂MA)-b-PMPC emulsifier stabilizing a water droplet of W/O emulsion, resulting in core–shell-structured microgels comprising PMPC core and P(OEGMA-co-MEO₂MA) shell. The resulting PMPC core–shell microgels dispersed stably in both chloroform and water without a thorough washing process. The transmittance of the aqueous PMPC core–shell microgel dispersion decreased drastically above 38 °C. The decrease in the transmittance of the PMPC core–shell microgel dispersion was attributed to the fact that the P(OEGMA-co-MEO₂MA) shell became hydrophobic above 38 °C. Because our method enables the facile encapsulation of various hydrophilic compounds into the core of temperature-responsive core–shell microgels, smart core–shell microgels have various potential applications, including smart drug delivery carriers and smart catalytic systems.