1 to y.Preferred metallic halides are aluminum halides of the formulawherein: each R is independently hydrocarbyl as hereinbefore defined such as alkyl, X is a halogen, and a is a number from 1 to 3.Most preferred are alkylaluminum halides such as ethylaluminum sesquichloride, diethylaluminum chloride, ethylaluminum dichloride, and diethylaluminum bromide, with ethylaluminum dichloride being espe