Catalytic hydrogenation of nitrile-butadiene copolymers by cationic rhodium complexes

Abstract

The hydrogenation of nitrile-butadiene copolymers (NBR), catalyzed by a series of cationic rhodium complexes [Rh(diene)(L 2)] + (diene=norbornadiene (NBD) and 1,5-cyclooctadiene (COD); L 2=(PPh 3) 2, Ph 2P(CH 2) n PPh 2 ( n=2, 3 and 4); Cy 2P(CH 2) 2PCy 2), was studied under 3.42 MPa hydrogen pressures at 140°C in monochlorobenzene. The general activity patterns found include: (1) the activity of complexes [Rh(diene)(PPh 3) 2] + is similar to that of Wilkinson catalyst RhCl(PPh 3) 3; (2) with chelating diphosphines, the activity of complexes [Rh(diene)(L 2)] + increases with the increase of the chain length of the diphosphine; (3) complexes [Rh(diene)(PPh 3) 2] + are more active catalysts than those containing chelating phosphines. These results demonstrate that there are a number of similarities and remarkable differences between the hydrogenation of NBR and the hydrogenation of simple olefins reported, which is mainly attributed to the steric effect of olefins. In contrast to simple olefin hydrogenation where the coordination of olefin to a metal center is typically facile, this step becomes rate-determining in the case of the hydrogenation of large olefins such as NBR. In order to determine the effects of the factors on the reaction, such as catalyst concentration, polymer concentration, hydrogen pressure and temperature, a detailed kinetic study on the reaction catalyzed by complex [Rh(NBD)(PPh 3) 2] + was carried out. On the basis of the kinetic results and the related statistical analysis, both a reaction pathway and a reaction rate law are proposed. The dilute solution viscosities of fully hydrogenated NBR were found to be independent of reaction conditions employed, suggesting that the properties of hydrogenated NBR are uniform.