A single zirconium-based constrained geometry catalyst (Me2Si(eta(1)-C29H36)(eta(1)-N-Bu-')ZrCl2 center dot OEt2/methylaluminoxane) has been employed to produce homopolymers of ethylene which contain degrees of branching unprecedented for early transition metal systems. A variable density of long branches is observed (10-50 branches per thousand carbon atoms) with strong dependences on both polymerization temperature and pressure. These branches are attributed to the exceptionally facile incorporation of long alpha-olefin macromonomers formed in situ via the usual beta-hydride elimination of polymer chains. Additionally, a relatively constant low degree of ethyl branches is observed (similar to 5 branches per thousand carbon atoms) under all polymerization conditions; this is attributed to P-hydride transfer to monomer followed by immediate reinsertion of the coordinated olefin-terminated polymer into the formed metal-ethyl bond.