1,3-Pentadiene cationic polymerization under the action of titanium tetrachloride-carboxylic acid (CF 3 COOH, CF 3 COOD, CCl 3 COOH, CHCl 2 COOH, CH 2 ClCOOH, CH 3 COOH, and (CH 3 ) 3 CCOOH) catalytic systems has been studied. The highest activity in the 1,3-pentadiene polymerization is shown by the TiCl 4 -trifluoroacetic or deuterated trifluoroacetic acids system. Poly-1,3-pentadiene synthesized over these catalytic systems is characterized by a relatively low M n , which changes slightly with an increasing monomer conversion, while M w of the polymer increases substantially. The polymer chain propagation rate constants and the concentration of active species at various initial 1,3-pentadiene concentration have been determined using a deuterium-containing initiator. The reduced unsaturation of poly-1,3-pentadiene synthesized via the cationic polymerization method is due to chain transfer to the polymer and does not result from the cyclization reaction with the formation of six-membered cyclic structures.