An in situ generated cationic allylpalladium complex bearing N-heterocyclic carbene (NHC) ligands, derived from the reaction of [eta(3)-C3H5)Pd(NHC)Cl] with AgX (X = BF4 or SbF6), is an active catalyst for the addition polymerization of norbornene and norbornene derivatives [5-norbornene-2-carboxylic acid methyl ester (b) and 5-norbornene-2-carboxylic acid n-butyl ester (c)] with an ester group containing a large portion of endo-isomers. The catalytic activities, polymer yields, molecular weights, and molecular weight distributions of polynorbornenes were investigated under various reaction conditions: the catalytic activity was highly dependent on the counteranion, the reaction solvent, and the reaction temperature. For b, as the portion of an endo-isomer increased, the activity of 13(SbF6) was much higher than those of 14 and 15, and for c (exo/endo = 95:5), the maximum turn over number (TON) was observed with 15(SbF6-). (c) 2007 Wiley Periodicals, Inc.