From among these examples, it is preferable to use organic aluminum compounds represented by the general formula AlR1 pR2 qX3???(p+q), wherein R1 and R2 each represent a hydrocarbon group such as an alkyl group, a cycloalkyl group or an aryl group, or an alkoxy group, R1 and R2 can be identical or different, X represents a halogen atom, and p and q are positive numbers satisfying the formula: 0<p+