Hydrophobic poly(N-[3-(dimethylaminopropyl)]methacrylamide-co-lauryl acrylate) [P(DMAPMA-coLA)l hydrogels with different LA content were synthesized by free-radical crosslinking copolymerization of corresponding monomers in water by using N,N-methylenebis(acrylamide) as the crosslinker, ammonium persulfate as the initiator, and N,N,N',N'-tetramethylethylenediamine as the activator. The swelling equilibrium of the hydrogels was investigated as a function of temperature and hydrophobic comonomer content in pure water. An interesting feature of the swelling behavior of the P(DMAPMA-co-LA) hydrogels with low LA content was the reshrinking phase transition where the hydrogels swell once and collapse as temperature was varied in the range of 30-40 degrees C. The average molecular mass between crosslinks (Mc) and polymer-solvent interaction parameter (chi) of the hydrogels were calculated from equilibrium swelling values. The enthalpy (Delta H) and entropy (Delta S) changes appearing in the chi parameter for the hydrogels were determined by using the Flory-Rehner theory based on the phantom network model of swelling equilibrium. The positive values for Delta H and Delta S indicated that the hydrogels had a positive temperature-sensitive property in water, that is, swelling at a higher temperature and shrinking at a lower temperature. (c) 2006 Wiley Periodicals, Inc.