A family of high activity catalysts for the vinyl addition polymerization of norbornene-type monomers based on cationic n(3)-allylpalladium complexes coordinated by phosphine ligands has been discovered. The palladium complex [n(3)-allyl)Pd(tricyclohexylphosphine)(ether)][B(3,5-(CF3)(2)C6H3)(4)] (2) was found to copolymerize 5-butylnorbornene and 5-triethoxysilylnorbornene (95:5 molar ratio) with truly high activity and is capable of producing more than a metric ton of copolymer per mole Pd per hour. Multicomponent catalyst systems based on the addition of salts of weakly coordinating anions (e.g., Na-[B(3,5-(CF3)(2)C6H3)(4)] or Li[B(C6F5)(4)].2.5Et(2)O) to (n(3)-allyl)Pd(X)(PR3) (X = chloride, acetate, nitrate, trifluoroacetate, and triflate) in the presence of norbornene-type monomers were developed. NMR tube experiments confirm that Na[B(3,5-(CF3)(2)C6H3)(4)] abstracts the Cl ligand from the palladium complex forming the cationic complex in situ. Control experiments confirmed that a high activity polymerization system requires a palladium cation containing an allyl ligand, a neutral, two-electron-donor phosphine ligand, and a weakly coordinating counterion. Those complexes where X contained electron-withdrawing groups such as trifluoroacetate or triflate were found to be the most active catalyst precursors. n(3)-Allylpalladium catalyst precursors with larger cone angle phosphine ligands yield lower molecular weight polymers. The poly(norbornene) molecular weights can be further tuned by addition of (x-olefin chain transfer agents to the reaction mixture. The catalyst systems were also found to polymerize norbornene-type monomers in aqueous media to high conversion at very low catalyst loadings. The effect of molecular weight on thermomechanical properties was explored.