Abstract: In a process for the in situ blending of polymers comprising contacting ethylene and one or more comonomers in two or more fluidized bed reactors with a catalyst system comprising (i) a magnesium/titanium based precursor containing an electron donor and (ii) a hydrocarbyl aluminum cocatalyst, the improvement comprising (A) increasing or decreasing the melt flow ratio and/or molecular weight of the blend by, respectively, decreasing or increasing the mole ratio of a precursor activator compound to the electron donor or (B) increasing or decreasing the bulk density of the blend by, respectively, increasing or decreasing the mole ratio of a precursor activator compound to the electron donor, both (A) and (B) subject to defined provisos including partial pre-activation of the precursor.