Methods for supplying a catalyst to an ethylene slurry loop polymerization reactor and polymers formed therefrom are described herein. The method includes preparing a catalyst slurry in a preparation vessel. The slurry includes a hydrocarbon diluent liquid that contains a Ziegler-Natta catalyst. The catalyst slurry is supplied to a buffer vessel, withdrawn therefrom and supplied to a slurry loop polymerization reactor in which ethylene is polymerized. A co-catalyst is mixed with the Ziegler Natta catalyst in the slurry prior to supplying the catalyst slurry to the slurry loop polymerization reactor. The transfer of the catalyst slurry from the preparation vessel to the buffer vessel and the withdrawal of catalyst slurry from the buffer vessel are controlled to maintain the level of catalyst slurry in the buffer vessel substantially constant relative to the level of catalyst slurry in the preparation vessel.