A catalyst precursor is formed by providing in slurry: (1) a carrier, which is porous and in the form of particles, or spheres, preferably particles of a crosslinked polymer, having a particle diameter of about 1 to about 300 microns, a porosity of about 0.001 to about 10 cc/gm, and a surface area of about 1 to about 1,000 m.sup.2 /gm; (2) a metallocene complex having an empirical formulaCp.sub.m MA.sub.n B.sub.pin which Cp is an unsubstituted or substituted cyclopentadienyl group, M is zirconium, titanium, or hafnium; m is 1, 2, or 3 and each of A and B is a halogen atom, hydrogen or an alkyl group, and (3) an anion forming reagent comprising either a borate complex or a borane. An alkyl aluminum compound is used in conjunction with the catalyst as either a cocatalyst or an impurity scavenger.