Synthesis and solution properties of fluorescently labeled amphiphilic (N-alkylacrylamide) oligomers

Amphiphilic poly(N-alkylacrylamide)s substituted at one chain end with the hydrophobic moiety N-n-octadecyl-[N-4-(1-pyrenyl)butyl] were prepared by end-group modification of carboxyl-terminated oligomers of N-isopropylacrylamide (NIPAM), N-(3-methoxypropyl)acrylamide (MPAM), and N,N-bis(2-methoxyethyl)acrylamide (BMEAM). The oligomers, M-n 2000-10000, were prepared by free-radical polymerization in the presence of mercaptopropionic acid acting as a chain-transfer agent. The oligomers were characterized by potentiometric titration of the end groups, H-1 NMR spectroscopy, and UV absorption measurements. Quasi-elastic light-scattering measurements and fluorescence experiments revealed the existence of oligomeric micelles, consisting of a hydrophobic core hosting pyrene aggregates captured within the octadecyl chain assembly, and a corona of solvated poly(N-alkylacrylamide) chains. The oligomeric PNIPAM micelles interact readily with hydrophobically modified PNIPAM,as evidenced by the following photophysical observations: (1) disappearance of pyrene excimer emission in favor of pyrene monomer emission and (2) relief of the pyrene/pyrene self-quenching prevalent within the intact micellar cores.