LCST in poly (N-isopropylacrylamide) copolymers: high resolution proton NMR investigations

A series of copolymers of N-isopropylacrylamide (NIPA) and acryloyl amino acids were synthesized and conjugated with p-aminobenzamidine (PABA). The values of lower critical solution temperatures (LCSTs) of these polymers estimated by 1H NMR were in agreement with those estimated by the turbidometric methods. The apparent shifts in the peak positions for the polymer with increasing temperature reported in the past are shown to be artifacts resulting from the deuterium lock experiment. The 1H-1H NOESY experiments were carried out to distinguish the interactions between the protons in: (a) 6-amino caproic acid (6ACA) and NIPA and (b) PABA and NIPA. These explain the variation in the LCST of the polymers based on the contributions of the two monomers to the magnitude of the polymer-polymer interactions. The findings are further supported by proton spin-lattice relaxation time measurements