Abstract: A process for preparing and controlling the M.sub.n of a reaction product comprising at least 90 mole % linear alpha olefins which includes polymerizing an ethylene containing gas in the presence of the reaction product of a Zirconium metal compound with R.sub.2 AlX, wherein R is an alkyl group having about 1 to about 5 carbon atoms and X is Cl or Br, in the presence of a diluent at a temperature of about 75.degree. to 200.degree. C. and an ethylene pressure above about 50 psia, wherein the mole ratio of ethylene to olefin reaction product is above 0.8 throughout the reaction, wherein the product olefin concentration is greater than 5 wt. % based on the diluent and reaction product, wherein the M.sub.n of said reaction product is controlled by the molar ratio of said R.sub.2 AlX/ZrCl.sub.4, said molar ratio being less than about 1.