Thermo-Responsive Diblock Copolymers of Poly(<i>N</i>-isopropylacrylamide) and Poly(<i>N</i>-vinyl-2-pyrroridone) Synthesized via Organotellurium-Mediated Controlled Radical Polymerization (TERP)

Diblock copolymers of poly(N-isopropylacrylamide) (PNIPAM) and poly(N-vinyl-2-pyrrolidone) (PNVP) (PNIPAMm-b-PNVPn) with well-defined block lengths were successfully synthesized by organotellurium-mediated controlled radical polymerization (TERP) based on the finding that the homopolymerization of N-vinyl-2-pyrrolidone was better-controlled by TERP than by macromolecular architecture designed by interchange of xanthates (MADIX), TERP resulting in a narrower molecular weight distribution of PNVP. Heat-induced association properties in water of three block copolymers with different block lengths, PNIPAM110-b-PNVP53, PNIPAM110-b-PNVP234, and PNIPAM76-b-PNVP219, were characterized by 1H NMR, turbidity, quasi-elastic light scattering (QELS), and static light scattering (SLS) techniques. All three block copolymers dissolve in water molecularly (as a unimer state) when the solution temperature is below an aggregation temperature (Ta) that is near a lower critical solution temperature (LCST) for the PNIPAM block. Comparing Ta between the two block copolymers of the same the PNIPAM block lengths (DPn = 110), the diblock copolymer with a shorter PNVP block length tend to associate at a lower temperature. On the other hand, the association occurred at a higher temperature for the diblock copolymers with shorter PNIPAM block lengths. When the temperature was raised above Ta, PNIPAM110-b-PNVP234 and PNIPAM76-b-PNVP219 formed apparently spherical core−corona micelles with aggregation numbers (Nagg) of 808 and 298, respectively at 60 °C. In contrast, PNIPAM110-b-PNVP53 formed a much larger aggregate with Nagg = 27 000. This aggregate was speculated to be a multi core aggregate formed by the association of individual core−corona micelles. The copolymers were found to be bound to gold nanoparticles in water through coordination interaction of the PNVP block with Au. The polymer coated gold nanoparticles indicated a temperature-dependent color change arising from a shift of the maximum wavelength of the plasomon band

Salt Effect on the Heat-Induced Association Behavior of Gold Nanoparticles Coated with Poly(<i>N</i>-isopropylacrylamide) Prepared via Reversible Addition−Fragmentation Chain Transfer (RAFT) Radical Polymerization

Poly(N-isopropylacrylamide) (PNIPAM) with a narrow molecular weight distribution was prepared by reversible addition−fragmentation chain transfer (RAFT) radical polymerization. A dithioester group at the chain end of PNIPAM thus prepared was cleaved by treating with 2-ethanolamine to provide thiol-terminated PNIPAM with which gold nanoparticles were coated via reactions of the terminal thiol with gold. The thermoresponsive nature of the maximum wavelength of the surface plasmon band and hydrodynamic radius (Rh) for the PNIPAM-coated gold nanoparticles were found to be sensitively affected by added salt. In pure water, Rh for the PNIPAM-coated gold nanoparticles at 40 °C (>lower critical solution temperature (LCST)) was smaller than that at 25 °C (Rh for the PNIPAM-coated gold nanoparticles at 40 °C was significantly larger than that at 25 °C. Heat-induced association and dissociation for the PNIPAM-coated gold nanoparticles were completely reversible in 50 mM NaCl aqueous solutions, which is responsible for the reversible thermoresponsive color change