A well-defined cyclic macromonomer based on a cyclic polystyrene was prepared, and its radical copolymerizations with vinyl monomers were carried out. The copolymerization with tert-butyl acrylate in benzene at 70 degreesC gave an insoluble cross-linked polymer with a good swelling property, The insoluble material was converted to soluble polymers by the cleavage of cyclic polystyrene from the backbone chain, suggesting a mechanically cross-linking process due to the threading of the cyclic polystyrene by a segment of another polymer chain during the copolymerization.