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