The ethylene polymerization and ethylene/alpha-olefin copolymerization behavior of the sterically crowded tris(pyrazolyl)borate group 4 metal complexes Tp(Ms*)TiCl(3) (1, Tp(Ms*) = HB(3-mesitylpyrazolyl)2(5-mesitylpyrazolyl)-), {K[Tp(Ms*)TiCl(3)]}(2) (2), Tp(Ms*)TiCl(2)(O-2,4,6-(BU3)-B-1-Ph) (3), Tp(Ms*)ZrCl(3) (4), Tp(Ms)ZrCl(3) (5, Tp(Ms) = HB(3-mesitylpyrazolyl)3(-)), and Tp(Ms*)HfCl(3) (6) were studied with dried-MAO activation in toluene solution. These catalysts all produce very high molecular weight (M-v > 10(6)) linear polyethylene. Zirconium catalyst 5 excels in productivity in ethylene homopolymerization and ethylene/ 1-hexene copolymerization, molecular weight, and hexene incorporation ability. Catalyst 5 produces moderately alternating ethylene/1-hexene copolymer (41 mol% hexene) with ultra-high molecular weight (M. = 1.3 x 10(6)), narrow molecular weight distribution (M-w/M-n = 2.2) and narrow composition distribution with high efficiency. (C) 2007 Elsevier B.V All rights reserved.