Rp h Rq i AlX3-p-q in which Rh represents a hydrocarbon group having 1 to 10 carbon atoms, preferably an alkyl group having 1 to 6 carbon atoms, an alkenyl group, a cycloalkyl group or an aryl group, Ri represents an alkoxy group having 1 to 6 carbon atoms or an aryloxy group, X is a halogen atom, and 3???p???0, and 2???q???0, may be used.