Use of alkylaluminum compounds and hydroxyl-containing 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, employing a supported chromium oxide catalyst which comprises

• (1) adding an alkylaluminum compound and a hydroxyl-containing compound to the reactor of a fluidized bed polymerization reaction system prior to the commencement of polymerization, and subjecting said alkylaluminum compound and said hydroxyl-containing compound to a pressure of from 350 kPa to 2000 kPa and a temperature of from 20.degree. C. to 150.degree. C. for a time sufficient for said alkylaluminum compound to react completely with the moisture present in said reactor and said hydroxyl-containing compound,
• said alkylaluminum compound having the formula
• AlR.sub.a R.sub.b R.sub.c
• wherein R.sub.a, R.sub.b and R.sub.c are alkyl radicals containing from 1 to 12 carbon atoms, and said alkylaluminum compound being added to said reactor in an amount sufficient to react with at least a major amount of the moisture present in the reaction system,
• said hydroxyl-containing compound having carbon-hydroxyl or metal-hydroxyl linkages which are capable of reacting with the alkylaluminum compound to form a Lewis salt, and said hydroxyl-containing compound being added to said reactor in an amount sufficient to react with all of the alkylaluminum compound added to the reactor, and
• (2) introducing a supported chromium oxide catalyst into the reactor under polymerization conditions suitable for said catalyst at a rate of from 5 percent to 100 percent of the rate normally employed during polymerization, and maintaining said conditions until polymerization is initiated.