Water-Soluble Copolymers .62. Nonradiative Energy-Transfer Studies of pH-Responsive and Salt Responsive Associations in Hydrophobically-Modified, Hydrolyzed Maleic Anhydride-Ethyl Vinyl Ether Copolymers

A series of labeled, hydrophobically modified polymers based on the alternating copolymer of maleic anhydride and ethyl vinyl ether (MAEVE) has been synthesized to specifically address the issue of differences in domain organization of closed, polysoap systems and open, associative-thickening systems. Modification was achieved by reaction of primary amines with MAEVE in organic solvent and hydrolysis in aqueous base. This unique method allows stoichiometic modification with hydrophobic and fluorescent groups. Intramolecular associations predominate in a wide range of environments. The extent to which these interactions occur is effectively probed by monitoring nonradiative energy transfer (NRET) between naphthalene and pyrene groups covalently bound to the polymer chain. Intrapolymer NRET quantum efficiency increases with decreasing pH as the chain contracts to a globular conformation. Interpolymer NRET is independent of salt concentration, but intrapolymer NRET steadily increases with [NaCl] as the shielding of charge-charge repulsions constricts the coil. In extremely acidic and basic environments, interpolymer NRET increases as electrostatic repulsions are shielded or eliminated. A mechanism has been proposed whereby the compaction of labeled polymer micelles results in an enhancement in interpolymer aggregation due to the formation of hydrophobic micellar surfaces