The effects of structure alteration of alkyl groups and addition of ions to solutions of new thermosensitive polymer series of poly(methyl 2-alkylamidoacrylate)s were investigated. Potential advantages of the thermosensitive polymers are their simplicity of functionalization and transition temperature control that result from their unique alpha,alpha-disubstituited structures. Poly(methyl 2-propionamidoacrylate) (PMPA) and poly(methyl 2-isobutyracrylate) (PMIBA) were thermosensitive polymers, and poly(methyl 2-acetamidoacrylate) and poly(methyl 2-n-butyramidoacrylate) were completely water-soluble and -insoluble, respectively. The PMIBA solution showed endotherm during the phase transition, while endotherm was not detected for PMPA. The difference between the two polymers resulted from the size of the hydrophobic groups. MPA gel was prepared by copolymerization with N,N'-methylenebis(acrylamide) and temperature-induced volume change of the gel was continuous. The salting-out effect of inorganic ions on PMPA solution and MPA gel followed the Hofmeister series. The inorganic (I- and SCN-) and organic ions ((Pr4N+)-Pr-n and (Bu4N+)-Bu-n), which showed the salting-in effect, were indicated to directly interact with PMPA chains. These ions widened the temperature range of the phase transition of the PMPA solutions. This reduced cooperativity of the phase transition was caused by size decrease of cooperative domains, which resulted from the interaction of the ions with the polymer. (c) 2005 Wiley Periodicals, Inc.