Preparation of cross-linked biocompatible poly(2-methacryloyloxyethyl phosphorylcholine) gel and its strange swelling behavior in water/ethanol mixture

Functional cross-linked poly(2-methacryloyloxyethyl phosphorylcholine(MPC)) gels were prepared with various cross-linkers by conventional radical polymerization in aqueous media. The poly(MPC) gels swelled in water and have a large degree of swelling compared with that of poly(2-hydroxyethyl methacrylate). The swelling degree of the poly(MPC) gel equilibrated in water depended on the chemical structure and composition of the cross-linkers, but did not change in an aqueous medium with pH and ionic concentrations. On the other hand, the volume of poly(MPC) gel changed sharply with a change in the composition of water/ethanol mixtures. Although in mixtures containing 70-90 vol% of ethanol the gel volume was nearly 10% of that in water, in pure ethanol it was almost the same as in water. The change in the volume of the poly(MPC) gel observed in a water/ethanol mixture was completely reversible. We conclude that this so-called re-entrant phenomenon was derived from the co-nonsolvency of poly(MPC) in the water/ethanol mixture triggered by an imbalance in the delicate interaction force between the molecules of water, ethanol, and poly(MPC).