. . . "Among the foregoing, preferred magnesium compounds are magnesium halides, alkoxy magnesium halides, aryloxy magnesium halides, alkoxy magnesiums and aryloxy magnesiums of the formula MgX2, Mg(OR5)X and Mg(OR5)2 (wherein X is halogen, and R5 is a hydrocarbon group such as an alkyl group or an aryl group optionally having a substituent such as a lower alkyl group)." . .