In-bulk photopolymerization of 2-acylamido-2-methylpropanesulphonic acid (AMPS) on gelatin matrix was made by ultraviolet irradiation at room temperature to form the crystalline and flexible membranes with giant spherulites (maximal radius: 2.2 cm). Number- and weight-average molecular weights of PAMPS were 79,500 and 138,000, respectively. The melting point of the membrane was around 200 degrees C. The giant spherulite formation was discussed on the basis of the PAMPS chain formation and PAMPS/gelatin complexation.