Homoleptic allyl complexes of divalent metals. M-II[1.3-C3H3(SiMe3)(2)](2) (1, M=Cr: 2. M=Fe: 3, M=Ni 4, M=Co) activated with methylaluminoxane (MAO) have been tested as catalysts for the polymerization of norbortiene. Whereas the iron and cobalt systems were poorly active, both the nickel and chromium complexes were very productive and gave high molecular weight poly(norbornene)s. although the polymers generated by Cr and Ni differed significantly in the degree of stereoregularity. Cr-II[1,3-C3H3(SiMe3)(2)](2) polymerized ethylene in the absence of any cocatalyst but, surprisingly, was deactivated by MAO. whereas 1/B(C6F5)(3) showed moderate activity at 20 degrees C under 1 bar of ethylene. NMR experiments suggest that B(C6F5)(3) acts as a one-electron oxidant to generate the [Cr-III(allyl)(2)](+) cation, Although ethylene/norbornene copolymerizations were possible with 1/B(C6F5)(3), the activity was much lower than in ethylene homopolymerizations, and only limited amounts of norbornene could be incorporated. The catalytic behavior of this Cr(II) precursor contrasts sharply with that of the Cr(IV) alkyl complex Cr(CH2SiMe3)(4) (5)/MAO which polymerizes ethylene but reacts with norbornene to give oligomers. mostly trimers-pentamers. On the other hand, (5)/MAO is highly active for ethylene/norbornene copolymerizations and gives high molecular weight copolymers. Significantly, catalyst activity increases with increasing norbornene concentration. The copolymers show block-structure, and incorporated norbornene is present at the least as di-norbornene units, even at incorporation levels as low as 10 mol%. At higher norbornene concentrations, NNN sequences prevail. (c) 2005 Elsevier B.V. All rights reserved.