Preparation and use of a new ziegler-natta catayst component

A method for the preparation of a supported procatalyst useful in the polymerization of olefins comprising:

• (a) forming particles of magnesium dihalide and alcohol,
• (b) reacting the particles with an organometal compound selected from the group consisting of compounds of the formula RM, wherein R is alkyl or aryl and M is lithium, sodium or potassium;
• R.sub.2 M', wherein R is alkyl or aryl and M' is zinc or cadmium;
• R.sub.2-1 MgX.sub.1, wherein R is alkyl or aryl, X is a halogen atom and 1 is 0 or 1;
• MAlR.sub.4, wherein M is lithium, sodium or potassium and R is alkyl or aryl;
• R'.sub.3-m AlX.sub.m, wherein R' is hydrogen, alkyl or aryl, X is a halogen atom and M is 0 or a positive number lower than 3;
• R'.sub.3-n Al(OR).sub.n, wherein R' is hydrogen, alkyl or aryl, R is alkyl or aryl and n is a positive number greater than 0 but smaller than 3;
• and RAl(OR)X, wherein R is alkyl or aryl and X is a halogen atom,
• (c) reacting the particle product of stage (b) with a titanium and/or vanadium compound and optionally and electron donor, and
• (d) optionally prepolymerizing the particle product of step (c),
• wherein said particles of magnesium dihalide and alcohol are formed by spray crystallizing a mixture of magnesium dihalide and alcohol to form crystallized complex particles.