RAFT-generated poly(N-isopropylacrylamide)-DNA block copolymers for temperature-responsive formation of polymer micelles

A diblock copolymer containing a single-stranded DNA and a temperature-responsive poly(N-isopropylacrylamide) (PNIPAAm-b-DNA) was synthesized. The PNIPAAm segment was obtained with a low polydispersity by reversible addition-fragmentation chain transfer polymerization. The number-averaged molecular weight and the molecular weight distribution of the PNIPAAm were determined with size exclusion chromatography. PNIPAAm-b-DNA was prepared through the Michael addition reaction of thiol-terminated PNIPAAm to 5′-maleimide-modified DNA. The lower critical solution temperature (LCST) of the copolymer was determined with differential scanning calorimetry. PNIPAAm-b-DNA self-assembled above the LCST into the formation of a polymer micelle with a hydrophobic PNIPAAm core surrounded by a hydrophilic DNA corona. Both light scattering and transmittance measurements revealed that the colloidal stability of the single-stranded DNA-carrying polymer micelles was enormously decreased when the complementary DNA was added into the micelle's dispersion to form the fully matched DNA duplex on the surface. It was also found that the colloidal stability of the micelle was extremely enhanced when the 5′-terminal-mismatched DNA was added to form a single-base mismatch at the interface between the DNA corona and the dispersal medium. The higher flexibility of the DNA terminus in the corona presumably increased the entropic repulsion between the micelles.