A family of high activity catalysts for the vinyl addition polymerization of norbornene-type monomers based on cationic η-allylpalladium complexes coordinated by phosphine ligands has been discovered. The palladium complex (η3-allyl)Pd(tricyclohexylphosphine)(ether)B(3,5-(CF3)2C6H3)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., NaB(3,5-(CF3)2C6H3)4 or LiB(C6F5)4·2.5Et2O) to (η3-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 NaB(3,5-(CF3)2C6H3)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. η3-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 α-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.