Process for the preparation of polyolefin

A process for the preparation of polyolefin by polymerization of alpha-olefins at a temperature from 50.degree. to 100.degree. C. and under a pressure of 3 to 12 bar in the presence of a catalyst composed of a spherical transition metal component (component A) and an organometallic compound (component B) in which the component A has been formed by reacting an organomagnesium compound with an organoaluminum compound, an electron donor, an organic chlorine compound and a transition metal compound, which comprises carrying out the polymerization in the presence of a catalyst in which the component A has been prepared by

• a) forming a solid composed of spherical particles having an average diameter of 20 to 110 .mu.m, a ratio of the mass average diameter D.sub.m, to the number average diameter D.sub.n is from 1.01 to 1.15 and a ratio of largest to smallest diameters ranging from 1.05 to 1.15 by reacting an organomagnesium compound of the formula R.sup.1 MgR.sup.2 in which R.sup.1 an R.sup.2 are identical or different alkyl radicals having 2 to 12 carbon atoms with an organoaluminum compound of the formula AlR.sup.3.sub.n (OR.sup.4).sub.3-n in which R.sup.3 and R.sup.4 are identical or different alkyl radicals having 1 to 8 carbon atoms and n denotes 0, 1, 2 or 3, or with the product from the reaction of aluminum trialkyls or aluminum dialkyl hydrides with diolefins containing 4 to 20 carbon atoms, and a primary, aliphatic chlorinated hydrocarbon, in an amount of 0.01 to 0.1 mol of the organoaluminum compound an 0.5 to 2.5 mol of the chlorinated hydrocarbon, relative to 1 mol of the organomagnesium compound, at a temperature from 30.degree. to 110.degree. C.,
• b) treating the resulting solid with an electron donor consisting essentially of
• i) oxygen-containing compounds of aluminum, silicon, phosphorus or sulfur, or
• ii) nitrogen or silicon compounds having alkyl or aryl radicals containing 1 to 8 carbon atoms, or
• iii) aliphatic ethers, in an amount of 0.1 to 1 mol per gram atom of the magnesium present in the solid, at a temperature from 0.degree. to 100.degree. C., and
• c) reacting the supporting material thus obtained with a compound of the formula MeXm (OR.sup.5) 4-m in which Me is Ti or Zr, R.sup.5 is an alkyl radical having 2 to 10 carbon atoms, X is a halogen atom and m is an integer from 0 to 4, added dropwise to said supporting material in an amount of 0.1 to 2 mol per gram atom of the magnesium present in the supporting material, and at a temperature from 30.degree. to 120.degree. C.