Abstract: Porous particles of MgCl2 having the form of essentially regular polyhedrons with six or eight faces in which the paired symmetrically opposite faces are essentially parallel, two of which faces are large and elongated and form the too face and the bottom face of a polyhedron such that on each of them the longest diagonal (D) is larger than the shortest distance (d) separating two opposite sides, which large elongated faces are surrounded essentially perpendicularly by the other essentially rectangular faces that form the sides of the said polyhedron, the length of the smaller side (e) of each of the said essentially rectangular faces being less than the shortest distance (d) separating the two opposite sides of the large elongated faces, catalytic components supported on the particles, catalyst systems utilizing the catalytic products, process for making the particles, and polyolefins obtained utilizing the catalytic systems.

Abstract: Porous particles of MgCl.sub.2 having the form of essentially regular polyhedrons with six or eight faces in which the paired symmetrically opposite faces are essentially parallel, two of which faces are large and elongated and form the top face and the bottom face of a polyhedron such that on each of them the longest diagonal (D) is larger than the shortest distance (d) separating two opposite sides, which large elongated faces are surrounded essentially perpendicularly by the other essentially rectangular faces that form the sides of the said polyhedron, the length of the smaller side (e) of each of the said essentially rectangular faces being less than the shortest distance (d) separating the two opposite sides of the large elongated faces, catalytic components supported on the particles, catalyst systems utilizing the catalytic products, process for making the particles, and polyolefins obtained utilizing the catalytic systems.

Abstract: A cocatalyst for use in association with a catalytic component comprising a titanium compound and an internal electron donor on a magnesium halide support for the polymerization of propylene alone or propylene with ethylene or a C.sub.4 to C.sub.12 alpha-olefin, said cocatalyst consisting essentially of an organic aluminum compound and an external electron donor consisting essentially of a non-aromatic silane with two Si--O--C bonds and a monoether and/or a silane with a single Si--O--C bond and the resultant catalyst wherein such cocatalyst is combined with a catalytic component known for use in polymerizing propylene.