The use of post-metallocene bis-phenylphenoxy catalysts to polymerize ethylene under high ethylene pressures (>25,000 psi) results in some remarkable catalytic properties. The high ethylene pressure produces molar ethylene concentrations in the reactor as much as 40 times higher than in typical low pressure ethylene polymerizations. This high ethylene concentration results in high catalyst efficiency at high temperatures and low reactor residence time, between 180 degrees C and 240 degrees C the catalyst efficiency surprisingly increases with increasing temperature, allowing for use of these catalysts at temperatures much higher than can be utilized in the low pressure processes. It has further been demonstrated that under these conditions increasing hydrogen levels up to 0.5 mol% does not significantly affect the polymer molecular weight; however, polymer molecular weight control can be realized with varying reactor temperature. The polymer produced is shown to be high density polyethylene made from a single site catalyst and not free radical initiated low density polymer. (C) 2016 Wiley Periodicals, Inc.