The invention is directed to organometallic catalysts prepared by a process comprising a) combining nucleophilic group-containing particulate support material with an arylboron or arylaluminum Lewis acid compound in the presence of a Lewis base compound; b) contacting the product of a) with a trialkylaluminum compound before combining said product with a metal precursor compound capable of activation for olefin polymerization by said product a); and, c) combining the product of b) with said metal precursor compound. These catalyst compositions are suitable for addition reactions of ethylenically and acetylenically unsaturated monomers. The invention includes a polymerization process of combining or contacting olefinically unsaturated monomers with the invention catalyst composition. Use of the invention catalyst to polymerize &agr;-olefins is exemplified.