The kinetics of the polymerization of isoprene in hexane in the presence of Ln naph-thenate-Al(i-Bu)3-Al2Et3Cl3 catalyst was studied in a specially designed dilatometer. The polymerization showed to be Ist-order in monomer and 1.75 th order in catalyst

respectively. In a typical polymerization

the concentration of propagating chain has been calculated to be 3×10-6mol/l. Thus

the efficiency of lanthanide catalyst is 2.8% and the rate constant of absolute propagations is 411/mol/sec (50℃).