Use of dialkylzinc compounds to initiate polymerization of ethylene with chromium oxide catalysts

A method for initiating homopolymerization of ethylene, or copolymerization of ethylene with at least one other alpha-olefin containing from 3 to 8 carbon atoms, in a fluidized bed reactor employing a supported chromium oxide catalyst which comprises (1) adding a dialkylzinc compound to the fluidized bed reactor prior to the commencement of polymerization and subjecting the reactor containing such dialkylzinc compound to a pressure of from 50 psi to 350 psi and a temperature of from 75.degree. C. to 115.degree. C. for at least 5 minutes, said dialkylzinc compound having the formula

• ZnR.sub.a R.sub.b
• wherein R.sub.a and R.sub.b are alkyl radicals containing from 1 to 12 carbon atoms, and said dialkylzinc compound being added to said fluidized bed reactor in an amount sufficient to provide from 1 mol to 5,000 mols of zinc per mol of chromium required to maintain a catalytically effective amount of such catalyst in the reactor during polymerization, and then (2) introducing a supported chromium oxide catalyst into the fluidized bed reactor at a rate of from 25 percent to 100 percent of the rate employed during polymerization to maintain a catalytically effective amount of such catalyst in the reactor, and maintaining said temperature and pressure until polymerization begins.