Graft copolymerization of styrene onto mercaptochitin has been examined. Chitin was first tosylated to give tosylchitin, which was subsequently transformed into mercaptochitin. Although the graft copolymerization was carried out in suspension, it proceeded efficiently to give chitin derivatives having polystyrene branches, a novel type of hybrid materials composed of a natural polysaccharide and a synthetic polymer. Under appropriate conditions, the grafting percentage reached 970%, indicating the high efficiency of the mercaptochitin as an initiator for the polymerization of styrene. The resulting graft copolymers exhibited glass transition phenomena at 115 degrees C and showed high swelling in organic solvents as a result of the introduction of polystyrene branches. Hydrolytic degradation of the chitin main chain allowed the isolation of the side chains, and the polystyrene isolated from the graft copolymer with a grafting percentage of 940% had M(n), M(w), and M(w)/M(n) values of 9.74 x 10(4), 2.55 x 10(5), and 2.62. These values indicate that the ratio of the mercapto groups actually used for initiating graft copolymerization was 4%, and a polystyrene chain attached on average to every 45 pyranose units.