Thermo- and pH-sensitive polymers containing cholic acid derivatives

New copolymers were prepared by free radical polymerization in solution from N-isopropylacrylamide and methacrylate monomers derived from cholic acid with ethylene glycol and oligo(ethylene glycol) spacers. These copolymers contained 1–5 mol% of the methacrylate derivatives of cholic acid. The lower critical solution temperature (LCST) of poly(N-isopropylacrylamide) in water was modified by small amounts of cholic acid-bearing comonomers since the bile acid residues tend to induce the aggregation of the polymers. In addition to the thermosensitivity, the copolymers also showed a response to pH changes when the carboxylic acid group of cholic acid was liberated by a selective hydrolysis of the ester protecting group. The addition of salt lowered the LCST of the polymers while the addition of surfactants raised it. The effects of a common surfactant, sodium dodecylsulphate, and that of a bile salt, sodium cholate, were compared. The chemical compositions, molecular weights and glass transition temperatures of the polymers have been determined. The glass transition temperatures of the copolymers were found to vary with their chemical compositions and the lengths of the spacer group.