CATALYTIC-HYDROGENATION OF NITRILE-BUTADIENE COPOLYMERS BY CATIONIC RHODIUM COMPLEXES

The hydrogenation of nitrile-butadiene copolymers (NBR), catalyzed by a series of cationic rhodium complexes [Rh(diene)(L-2)](+) (diene = no rbomadiene (NBD) and 1,5-cyclooctadiene (COD); L-2 = (PPh3)(2), Ph2P(C H2)(n)PPh2 (n = 2, 3 and 4); Cy2P(CH2)(2)PCy2), was studied under 3.42 MPa hydrogen pressures at 140 degrees C in monochlorobenzene. The gen eral activity patterns found include: (1) the activity of complexes [R h(diene)(PPh3)(2)](+) is similar to that of Wilkinson catalyst RhCl(PP h3)(3); (2) with chelating diphosphines, the activity of complexes [Rh (diene)(L-2)](+) increases with the increase of the chain length of th e diphosphine; (3) complexes [Rh(diene)(PPh3)(2)](+) are more active c atalysts than those containing chelating phosphines. These results dem onstrate that there are a number of similarities and remarkable differ ences 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 coordina tion of olefin to a metal center is typically facile, this step become s rate-determining in the case of the hydrogenation of large olefins s uch as NBR. In order to determine the effects of the factors on the re action, such as catalyst concentration, polymer concentration, hydroge n pressure and temperature, a detailed kinetic study on the reaction c atalyzed by complex [Rh(NBD)(PPh3)(2)](+) was carried out. On the basi s of the kinetic results and the related statistical analysis, both a reaction pathway and a reaction rate law are proposed. The dilute solu tion viscosities of fully hydrogenated NBR were found to be independen t of reaction conditions employed, suggesting that the properties of h ydrogenated NBR are uniform. (C) 1998 Elsevier Science B.V. All rights reserved.