Several cationic (allyl)Ni(II) complexes were synthesized and shown to be highly active for (2,3)-vinyl addition polymerization of norbornene to yield polymers with low molecular weight distributions (MWDs) ranging from 1.4-1.9. In all cases slow initiation was followed by rapid propagation which prevents molecular weight control of the poly(norbornene). One of the intermediates in the polymerization process has been identified and characterized by NMR spectroscopy as the first insertion product resulting from the insertion of norbornene into the Ni-C allyl bond in cis-exo fashion. This insertion product was synthesized independently and NMR studies showed that the first insertion of norbornene into the Ni-C ally] bond is a reversible process. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 2560-2573, 2009