The polymerization behavior of novel polynuclear cage complexes as precatalysts in the vinyl or addition polymerization of norbornene has been investigated and correlated with the results of the known mononuclear precatalysts M(acac), (M = Ni-II, Co-III, Cr-III, and Fe-III, acac = acetylacetonate, x = 2 or 3) and mixtures thereof. The cage complexes can be activated with the Lewis acids methylalumoxane (MAO) as well as with tris(pentafluorophenyl)borane, B(C6F5)(3), in combination with or without triethylaluminum (AlEt3). It is shown that nickel is the most active metal in the polymerization of norbornene with heterometallic precatalysts. The homo- and heterometallic nickel cage compounds reveal a maximum activity per nickel in the presence of "inert metal" atoms-an effect which is not seen when Ni(acac)(2) is only physically mixed with other metal-acac salts. The inert metal effect may be advantageously applicable to other valuable catalysts when used as heterometallic polynuclear compounds. (C) 2003 Elsevier Inc. All rights reserved.