Olefin polymerisation catalyst prepn. - comprises treating modified magnesium halide carrier with soln. of active hydrogen-contg. organic cpd

Mixed catalyst for polymerising 1-olefins of formula R3-CH=CH2 (I) (where R3 is a 1-8C alkyl) are prepd. by modifying an Mg halide with an electron donor and/or a cyclopolyene, reacting the modified carrier (II) with a halogen-contg. Ti cpd. and mixing the catalyst component obtd. with an organic Al cpd. and with a stereo-regulator. Improvement consists of treating the modified carrier with a soln. of >=1 active H atom-contg. organic cpd. (III) before reacting with the halogen-contg. Ti cpd. The catalysts are used for homo-polymerising (I); for co-polymerising (I) with one another and/or with =6 wt.% C2H4 or for block polymerising (I) with one another and/or with =35 wt.% C2H4, and partic. for the prepn. of isotactic polypropylene. (I)-Polymerisation is stereospecific. Catalyst removal is unnecessary. Finely powdered polypropylene prodn. is minimised, e.g. to 5 wt.%. Polymerisation results can be repeated.