The invention relates to a process for transitioning from a first contiunuous polymerization reaction of ethylene and a first comonomer for producing a linear low density polyethylene conducted in the presence of a Ziegler-Natta catalyst in a gas phase reactor to a second contiunuous polymerization reaction of ethylene and a second comonomer for producing a high density polyethylene conducted in the presence of a chromium-based catalyst in the gas phase reactor, the process comprising: (i) reducing the feed of the first comonomer into the reactor until the ratio of the first comonomer to ethylene in the reactor is at most 0.1; (ii) discontinuing the introduction of the Ziegler-Natta catalyst while the introduction of a co-catalyst of the Ziegler-Natta catalyst is continued and subsequently discontinuing the introduction of the co-catalyst; (iii) maintaining the polymerization conditions in the reactor and permitting polymerization to continue for a time in order to allow the components of the Ziegler-Natta catalyst present in the reactor to consume themselves in the production of additional polymer; (iv) discontinuing the introduction of all feeds into the reactor; (v) depressurizing the reactor; (vi) flow-purging the reactor; (vii) reducing the reactor temperature; (viii) introducing ethylene and H.sub.2 into the reactor to obtain a partial pressure of ethylene and a volume ratio of H.sub.2 to ethylene for the second polymerization reaction, wherein the partial pressure of ethylene is increased to the pressure for the second polymerization reaction at such a speed that the reactor temperature is maintained at a temperature lower than the temperature of the first polymerization conditions; (ix) increasing the reactor temperature to a temperature of the second polymerization conditions; (x) introducing the second catalyst into the reactor and (xi) introducing the second comonomer into the reactor to obtain a reactor composition for the second polymerization reaction.

Embodiments of methods of producing ethylene-based polymer comprising first and second polymer fractions includes reacting ethylene monomer and optionally C.sub.3-C.sub.12 -olefin comonomer in solvent in the presence of a first catalyst in at least one initial reactor to produce the first polymer fraction reaching an exit temperature of this reaction zone below 160 C., wherein the weight averaged molecular weight (Mw) of this first polymer fraction is larger than 500,000 g/mol; introducing the first polymer fraction, ethylene monomer, optionally C.sub.3-C.sub.12 -olefin comonomer, solvent, at least one second catalyst to at least one agitated solution polymerization reactor; and reacting the ethylene monomer and optionally C.sub.3-C.sub.12 -olefin comonomer in solvent in the presence of the at least one second catalyst in the at least one agitated solution polymerization reactor to produce a second polymer fraction.