The effect of an introduction of phosphate groups into hydrogels on th
eir stimuli-sensitivities has been investigated. Several hydrogels con
taining various phosphate group content were synthesized via solution
copolymerization of 2-(methacryloyloxy)ethyl dihydrogen phosphate (Pho
smer) and 2-hydroxyethyl methacrylate (HEMA) in the presence of 2,2'-a
zoisobutyronitrile (AIBN) in methanol at 70-degrees-C for 4 h. The con
tent of phosphate groups incorporated into the resulting hydrogels ran
ged from 1 to 13 mol-%. The swelling ratio of these hydrogels was foun
d to be strongly affected by pH, solvent, temperature and phosphate gr
oup content. The polymer-solvent interaction parameter chi of the hydr
ogel was determined from the Flory-Rehner equation, using the swelling
ratio and the crosslinking density derived from the compression modul
us. Furthermore, chi was divided into its enthalpic (chi(H)) and entro
pic component (chi(S)). Using the interaction parameter, the stimuli-s
ensitivity of the hydrogel is discussed in detail based on thermodynam
ic aspects.