Embodiments in accordance with the present invention encompass methods of forming in situ olefin polymerization catalyst systems, catalysts encompassed by such systems and polymers made using such systems. For such in situ olefin polymerization catalyst systems, a hydrocarbyl magnesium halide is generally contacted with a halohydrocarbyl compound to form a halohydrocarbyl Grignard and such Grignard is generally contacted with a Group 10 metal compound to form an olefin polymerization catalyst which is contacted with one or more olefin monomers to form a polymer therefrom.