Temperature-induced phase transition of well-defined cyclic poly(N-isopropylacrylamide)s in aqueous solution

The cyclic PNIPAMs were synthesized by ring closure of α,Σ- heterodifunctional telechelic PNIPAM precursors. The synthesis of PNIPAM was accomplished in three steps, first telechelic PNIPAMs were synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization of N-isopropylacrylamide (NIPAM) using an azide containing trithiocarbonate, 2-(2-azidoethoxy)ethyl 2-( 1-isobutyl)sulfanylthiocarbonylsulfanyl-2-methyl propionate) (AIP) as chain transfer agent. A low initiator/AIP (1:10) ratio was selected in order to ensure high terminal functionality and narrow molecular weight distribution. The structure of the 1-PNIPAM samples was confirmed by analysis of their 1H NMR spectra and the aqueous solutions of c-PNIPAM and its linear precursors were analyzed by high sensitivity differential scanning microcalorimetry and turbidity.