Compositions exhibiting high catalytic activity in the polymerization of .alpha.-olefins are provided by reacting a dinitrogen and/or dihydrogen complex of a transition metal such as titanium with an organometallic compound of a divalent metal such as dihydrocarbyl magnesium or hydrocarbyl magnesium halide and a halide source such as alkyl aluminum halide. Polymerization processes employing this catalyst composition do not require conventional catalyst removal steps in order to provide polymers having suitable color and other physical characteristics.