A new technique for the determination of the concentration of active centers of polymerization and propagation rate constants for processes of cationic polymerization of conjugated dienes was proposed. The method consists of the quantitative calculation from the 13 C NMR spectra of the concentration of the initial units of the polymer chain polydienes synthesized at different monomer conversions and the subsequent extrapolation of the obtained values to the zero monomer conversion. Using the proposed procedure, the chain propagation rate constants were determined for the cationic polymerization of isoprene and 1,3-pentadiene in the presence of a titanium tetrachloride– tert -butyl chloride catalytic system.