Pd(dibenzylideneacetone)(2), when activated in situ with 1 equiv of [CPh3][B(C6F5)(4)] in the presence of 1 equiv of P(C6H11)(3), efficiently catalyzed the addition polymerization and copolymerization of norbornene and its derivatives. Homopolymerization of 5-vinyl-2-norbornene took place regio-selectively with the endo-cyclic double bond to give high-molecular weight polymers, while the exocyclic double bond remained intact so that the resulting polymer had pendent vinyl groups along the polymer chain. In the polymerization of a mixture of the endo-/exo- isomers of 2-methoxycarbonyl-5-norbornene, the endo-isomer was consumed prior to consumption of the exo-isomer, contrary to the well-known tendency in Pd(II)-based catalyst systems. Another notable feature of the present catalytic system was the strong dependency of the molecular weight on the reaction temperature, which was studied in detail for the copolymerization of 2-methoxycarbonyl-5-norbornene with norbornene: we could control the molecular weight without the use of a chain transfer agent. The extracted oligomeric fraction of poly(norbornene) showed the presence of a terminal CC double bond as well as a C6F5 unit that was bound to the first norbornane unit in the polymer chain. (C) 2011 Elsevier Ltd. All rights reserved.