The mechanical properties of atactic 1,2-polybutadiene and 3,4-polyisoprene can be modified significantly with the addition of bis(acetonitrile)dichloropalladium(II). These weak rubbery polymers are transformed into glassy materials when the salt concentration is approximate to 4 mol %, in the absence of high-temperature annealing. Stress-strain measurements and Fourier transform infrared (FTIR) spectra for blends of cis-polybutadiene and PdCl2, without high-temperature annealing, suggest that pi-complexes form between palladium and the olefinic groups within the backbone of the polymer. These solid complexes cannot be dissolved in the original solvent (tetrahydrofuran), nor can they be disrupted by triphenylphosphine. Young’s modulus of the cis-polymer is enhanced by a factor of 50 when the salt concentration is 4 mol %, and the fracture strain is approximately 300%. An exothermic process centered at approximate to 250 degrees C accompanied by minimal weight loss suggests that PdCl2 could trigger high-temperature dimerization reactions of the carbon-carbon double bonds in the backbone of the cia-polymer. High-temperature annealing effects on the stress-strain response of cis-polybutadiene with 4 mol % PdCl2 are consistent with the data from calorimetry, suggesting that catalytically induced chemical crosslinking is operative at high temperatures. This latter claim is verified by infrared spectroscopy at ambient and elevated temperatures. Hence, bis(acetonitrile)dichloropalladium(II) coordinates to and catalyzes dimerization reactions of olefinic groups when they are present in the main chain or the sidegroup. This square-planar transition-metal salt also enhances the high-strain mechanical response of commercial styrene-butadiene-styrene triblock copolymers (Kraton(TM) D series). Reactive blending and compatibilization with transition-metal salts are attractive strategies to modify the mechanical properties of commercially important diene-based polymers that contain unsaturation in the main chain or the sidegroup.