Synthesis and characterization of photoresponsive N-isopropylacrylamide cotelomers

Free radical chain transfer polymerization of N-isopropylacrylamide (NIPAM) and three different succinimide-bearing comonomers was used to synthesize semitelechelic cotelomers characterized by a statistical distribution of the comonomers, low polydispersity, and controlled molar mass. Upon temperature increase, aqueous solutions of the cotelomers underwent a reversible phase transition (precipitation) once the critical solution temperature was surpassed. The critical solution temperature of the cotelomers showed characteristic differences to the one observed for PNIPAM homopolymers. A polymer analogous route exploiting the N-hydroxysuccinimide groups of the cotelomers was then used to introduce light-responsive azobenzene groups into the molecules. By irradiation (305 nm), the azo group is switched from the more hydrophobic trans to the more hydrophilic cis state. The half-life of the cis state in the dark was approximately 13 h. The influence of the temperature, the pH, and irradiation on the temperature-induced phase transition was studied by turbidimetry, UV-vis spectroscopy, and microcalorimetry. The observed shifts of the critical solution temperature were determined not only by the hydrophobic/hydrophilic balance and the chemical microstructure of the cotelomers but also by their conformation in solution. Whereas most cotelomers showed the expected increase of the critical solution temperature upon irradiation, at least one cotelomer was found with a lowered critical solution temperature in the irradiated state.