Star-shaped and star-block polymers with a porphyrin core: from LCST–UCST thermoresponsive transition to tunable self-assembly behaviour and fluorescence performance

Abstract

Star-shaped copolymer poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA) with a porphyrin core (THPP(-PDMAEMA-Br)₄) was synthesized by atom transfer radical polymerization (ATRP). After reaction with excess NaN₃, THPP(-PDMAEMA-Br)₄ was transformed to THPP(-PDMAEMA-N₃)₄. Star-block copolymer THPP(-PDMAEMA-b-PEG)₄ was obtained by click reaction of THPP(-PDMAEMA-N₃)₄ and alkynyl poly(ethylene glycol) (alkynyl PEG). After reaction of the PDMAEMA segment with excess 1,3-propane sultone, the quaternized THPP(-PDMAEMA-Br)₄ (THPP(-PDMAPS-Br)₄) and quaternized THPP(-PDMAPS-b-PEG)₄ were obtained. These copolymers can self-assemble into spherical micelles by directly dissolving in water. The thermoresponsive micelle solutions showed transition from a lower critical solution temperature (LCST) of THPP(-PDMAEMA-Br)₄ and THPP(-PDMAEMA-b-PEG)₄ to an upper critical solution temperature (UCST) of THPP(-PDMAPS-Br)₄ and THPP(-PDMAPS-b-PEG)₄, indicating that the LCST–UCST transition of the micellar solutions can be accomplished by the transition of PDMAEMA to PDMAPS. Furthermore, the presence of permanent hydrophilic PEG chains in star-block copolymers changed the values of LCST, UCST and hydrodynamic radius (R_h) of micelles. The micelle solutions presented obvious fluorescence performance. The fluorescence intensity decreased with the increase of temperature, indicating that the variation of temperature could alter the fluorescence intensity of these copolymer micelle solutions.