A process for the polymerisation of olefins is described involving the use
of a catalyst system comprising a supported transition metal compound and
an activator based on an organoborane and an aluminoxane. Preferably the
transition metal compound is a metallocene complex. The support is
pretreated with an organoaluminium compound and the process is
particularly suitable for the gas phase preparation of copolymers of
ethylene and alpha-olefins having a molecular weight distribution in the
range 2.5-7.0 and a melt strength in the range 3-12 cN.