R1 m Al(OR2)n Hp Xq wherein R1 and R2 are each a hydrocarbon group having usually 1 to 15 carbon atoms, preferably 1 to 4 carbon atoms, R1 and R2 my be the same or different from each other, X is a halogen atom, and m, n, p and q are numbers satisfying the conditions of 0<m???3, 0???n<3, 0???p<3, 0???q<3 and m+n+p+q=3;