Polymerization processes are disclosed for producing polyethylene
having a target density and improved environmental stress resistance. Ethylene
and optionally one or more -olefin comonomers, supported chromium catalyst,
and metal alkyl cocatalyst are contacted to produce polyethylene. The density of
the polyethylene is determined, and the concentration of cocatalyst is adjusted
in response to a deviation between the density of the polyethylene and the target
density. The catalyst and cocatalyst can be fed separately into the reactor, or
can be co-fed, eliminating the need to pre-contact the catalyst and cocatalyst
prior to introducing them into the reactor.
