d compound having the formula RMgX, in which R is an alkyl, cycloalkyl or aryl radical containing from 2 to 16 carbon atoms, and X is Cl or Br, in an aliphatic hydrocarbon, at a temperature between 35??? and 40??? C. and a Grignard compound/titanium compound molar ratio equal to at least 7, isolating the solid reaction product, and reacting it with a halogenating agent at a temperature between 0??