Hydrogenation is a useful method which has been used to improve oxidative and thermal degradation resistance of diene-based polymers. The quantitative hydrogenation of cis-1,4-polyisoprene which leads to an alternating ethylene-propylene copolymer was studied in the present investigation. To examine the influence of key factors on the reaction, such as catalyst concentration, polymer concentration, hydrogen pressure, and temperature, a detailed study of the hydrogenation of cis-1,4-polyisoprene catalyzed by the Ru complex, Ru(CH=CH(Ph))Cl(CO)(PCy3)(2) was carried out by monitoring the amount of hydrogen consumed. Infrared and H-1-NMR spectroscopic measurements confirmed the final degree of hydrogenation. The hydrogenation of cis-1,4-polyisoprene followed pseudo-first-order kinetics in double-bond concentration up to high conversions of double bond, under all sets of conditions studied. The kinetic results suggested a first-order behavior with respect to total catalyst concentration as well as with respect to hydrogen pressure. The apparent activation energy for the hydrogenation process, obtained from an Arrhenius plot, was 51.1 kJ mol(-1) over the temperature range of 130 to 180degreesC. Mechanistic aspects of the catalytic process are discussed. (C) 2004 Wiley Periodicals, Inc.