Hydrogels containing phosphate groups were prepared by copolymerization of 2‐methacryloyl‐oxyethyl dihydrogen phosphate (phosmer) and various hydrophilic monomers [ N , N ‐dimethyl‐acrylamide (DMAAm), acrylic acid (AAc) and 2‐hydroxyethyl methacrylate (HEMA)], and the swelling behavior was investigated. These hydrogels are thermo‐sensitive. Phosmer‐DMAAm and phosmer‐HEMA hydrogel deswell with increasing temperature, but for the phosmer‐AAc hydrogel the swelling ratio increases with temperature. Interestingly, the swelling ratio decreases with an increase in phosphate group content. This unusual behavior may arise from the phosphate group acting both as the functional group and the crosslinking agent.
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Nakamae et al. (1992) studied this question.