Preparation of spherical magnesium alkoxide particles

A process for polymerizing olefins which comprises:

• (a) first preparing spherical particles of a compound with the general formula Mg(OR).sub.2-a (OR').sub.a in which R is an alkyl group with from 2 to 8 carbon atoms, R' is an alkyl group with from 1 to 7 carbon atoms and a is a number of from 0 to 0.5 which process comprises:
• (i) preparing a homogeneous solution of a compound Mg(OR).sub.2 in an aliphatic alcohol with the general formula R'OH.
• (ii) spray-drying the resulting solution at a temperature of from 15.degree. to 85.degree. C. therein forming solid particles,
• (iii) suspending the spray-dried solid particles in an aliphatic alcohol with the formula ROH in which R is the same alkyl group as present in the said compound Mg(OR).sub.2,
• (iv) heating the resulting suspension in order to remove an alcohol with the formula R'OH from the liquid phase by distillation, and
• (v) separating the solid particles from the liquid phase,
• (b) halogenating the spherical particles produced in (a) with a halide of tetravlaent titanium in the presence of a halohydrocarbon and an electron donor, the halogenation optionally being preceding by converting the spherical Mg-alkoxide particles into a compound with the formula Mg(OR).sub.2-b X.sub.b, in which b is a number of from 0.3 to 1.8 and X is halogen, by reacting with a halogenating agent other than a titaniumhalide, and
• (c) contacting the halogenated spherical particles with at least one alpha olefin having at least 3 carbon atoms in the presence of an organoaluminum compound.