Catalytic hydrogenation of diene polymers: Part II. Kinetic analysis and mechanistic studies on the hydrogenation of styrene—butadiene copolymers in the presence of RhCl(PPh 3) 3

Abstract

Homogeneous catalytic hydrogenation of unsaturated polymers is a useful method of altering the chemical and physical properties of the parent polymer. Presented here is a detailed kinetic study of the hydrogenation of styrene—butadiene copolymers (AB and ABA block) using Wilkinson's catalyst, RhCl(PPh 3) 3. The effects of polymer, hydrogen, catalyst and added PPh 3 concentration, as well as temperature, on the rate of hydrogenation were examined. From this information a rate law has been developed for which the associated rate and equilibrium constants were estimated. These rate constants are compared with those previously obtained for other polybutadiene systems. The reaction kinetics provide a further understanding of the RhCl(PPh 3) 3 catalysed mechanism of hydrogenation of polymers. The steric effect of the aromatic ring in the styrene—butadiene copolymer chain and the compatibility of the polymer with the solvent are important factors governing the rate of hydrogenation. Quantitative hydrogenation was confirmed by IR spectroscopy and 1H NMR. Gel permeation chromatography suggests no significant change in the chain length of the polymer upon hydrogenation.