Abstract: A aliphatic hydrocarbon soluble mixed aluminum hydrocarbyl-Lewis base co-catalyst composition comprising (i) at least two different classes of aluminum hydrocarbyls the first being an aluminum hydrocarbyl halide, the second being an aluminum hydrocarbyl hydride and/or aluminum trialkyl and (ii) at least one Lewis base represented by the formula: ##STR1## wherein each Y can be -OR', -NR.sub.2 ' and R' with the proviso that at least two be -OR' or -NR.sub.2 ', Z is an alkylene radical forming a 5 or 6 membered ring with the carbonyldioxy group or Z is alkyl substituted alkylene group wherein the alkyl radical can have from 1 to 8 carbon atoms, R is an aryl, allyl or alkyl group having 1 to 20 carbon atoms and R' is an alkyl, cycloalkyl, aryl or aralkyl radical having from 1 to 20 carbon atoms.The co-catalyst is employed with a titanium halide catalyst for the polymerization of olefins to obtain a advantageous balance of catalytic activity and polymer product stereoregularity.

Abstract: Polyolefins are prepared wherein at least about ninety-five percent of the product is retained on a 100 mesh screen. The polymerization process involves contacting the alpha-olefin with a Ziegler catalyst system wherein the catalyst has been prepared by mixing the active catalytic particles with a fibrillatable polyolefin and a non-fibrillatable polyolefin followed by ballmilling the mixture for a period of time sufficient to cause a fibrous mat to form, entrapping the catalyst particles in the mat which is, in turn, carried by the non-fibrillatable polyolefin.