atoms and X is a halogen atom, and/or a dialkyl magnesium compound represented by the general formula R7R8Mg wherein R7 and R8 are each a hydrocarbon group having 1 to 20 carbon atoms and a polymeric support consisting of polyvinylchloride, polyvinylalcohol, polyketone, hydrolyzed polyketone, ethylene-vinylalcohol copolymer or polycarbonate; and