Abstract <p>A series of catalytic systems based on palladium complexes with phosphine-thioether ligands has been investigated in the additive polymerization of norbornene and its derivatives. The effects of the solvent nature and monomer concentration on the yield of the additive polynorbornenes synthesized in the presence of pre-catalysts of the (P,S)Pd(CH₃)Cl structure, activated using AgSbF₆, have been established. Cationic palladium complexes of the [(P,S)Pd(CH₃)(L)]⁺(X)⁻ composition (where L = CH₃CN, DMSO, or 3,5-lutidine; X = BF₄, SbF₆) have been synthesized for the first time and characterized by means of ¹H, ¹³C, and ³¹P NMR spectroscopy. Additive polymerization of norbornene in the presence of these cationic complexes has been investigated. The effect of the labile ligand and the counterion on the activity of the cationic palladium phosphine-thioether complexes in the additive polymerization of norbornene has been established. The activity of palladium cationic complexes in additive polymerization is significantly affected by the addition of B(C₆F₅)₃, which binds the ligand L and promotes the dissociation of the complex into its active form.</p>