Multi-responsive Hyperbranched Star Copolymer: Synthesis, Self-assembly and Controlled Release

Multi-responsive (temperature, pH and redox) hyperbranched star polymers, poly(2-(2-methoxyethoxy)ethyl methacrylate)-star-poly(dimethylaminoethyl methacrylate) (H-PMEO(2)MA-star-PDMAEMA) have been successfully synthesized by self-condensing vinyl polymerization of disulfide-based inimer and MEO(2)MA first, and subsequently atom transfer radical polymerization of DMAEMA with H-PMEO(2)MA as macroinitiator. The M-n and M-w/M-n of the H-PMEO(2)MA were 8300 g/mol and 2.61, respectively. H-PMEO(2)MA-star-PDMAEMAs with different molecular weights were obtained by adjusting the polymerization time. The molecular weight of the hyperbranched star copolymer increased but the polydispersity index (PDI) decreased with increasing polymerization time. Since the PDI of the PDMAEMA formed by ATRP is low, with the molecular weight increase of the PDMAEMA, the relative amount of H-PMEO(2)MA in the hyperbranched star copolymers becomes less; as a result, the influence of the core H-PMEO(2)MA's PDI on the hyperbranched star copolymers decreases. UV/Vis TU-1901 spectrophotometer was used to investigate the lower critical solution temperature (LCST) of the resultant polymer. The LCST of H-PMEO(2)MA is relatively low (2-10 degrees C). The effects of the compositions and pH of the solution on LCST of the hyperbranched star copolymers were studied. The LCST increased with the chain length increase of PDMAEMA. The pH of the solution has a significant impact on the LCST of the hyperbranched star copolymers. With decrease of the pH value, the protonation degree of PDMAEMA increased, the repulsion between the chain segments enhanced, making the aggregation of the H-PMEO(2)MA-star-PDMAEMA molecules become difficult, and as a result, the water-solubility of the hyperbranched star copolymers enhanced. In addition, when temperature of the aqueous solution raised from 2 degrees C to room temperature, the spherical micelles with H-PMEO(2)MA as core and PDMAEMA as shell were formed. During the formation of spherical micelles in the aqueous solution of H-PMEO(2)MA-star-PDMAEMA and Nile Red, the Nile Red was successfully encapsulated in the micelles. The controlled release of this system, in which Nile Red was used as model drug, was investigated, the results showed that this system is pH and redox-responsive, and may have potential application in drug delivery.