A comparative study of the performances of (bpzmp)ZrBZ(3) (1) and (hppzm)ZrBz (2) activated with B(C6F5)(3) or [Ph3C]B(C6F5)(4)] n ethylene polymerisation pointed out that the steric properties of the metal complex and the boron activator affect the productivity of the catalyst and, more remarkably, the microstructure of the polyethylenes produced. A polymerisation mechanism accounting for these results has been proposed.