--as organo-metallic compounds of a metal of groups II and III of the periodic table, use can be made of one or more organo-aluminum compounds having the mean formula AlRx Z.sub.(3-x) wherein R represents an alkyl group containing from 2 to 12 carbon atoms, Z represents a hydrogen atom or a halogen, such as chlorine or bromine, and x represents an integer or a fraction which can be of any value fr