It is well known that the carbon-carbon double bonds in a nitrile rubber, the nitrile rubber being a polymer comprising a C.sub.4 -C.sub.6 conjugated diolefin and a C.sub.3 -C.sub.5 unsaturated nitrile, can be selectively hydrogenated, without significant hydrogenation of the C.dbd.N bonds, by treatment of the polymer with hydrogen in the presence of selected catalysts--for example, British Patent 1,558,491; U.S. Pat. Nos. 3,700,637; 4,384,081; 4,464,515; and 4,503,196. The use of ruthenium catalysts for the hydrogenation of nitrile rubbers is described in U.S. Pat. Nos. 4,631,315; 4,816,525, 4,812,528 and 5,057,581. The use of certain additives useful in the ruthenium catalysed hydrogenation of nitrile rubbers is described in U.S. Pat. No. 5,075,388.
In the hydrogenation of nitrile rubbers, it has been found that, depending on the nature of the catalyst, the solvent used in the hydrogenation process and the reaction conditions used for the hydrogenation, the molecular weight of the hydrogenated nitrile rubber increases during the hydrogenation process. The molecular weight increase is believed to be due to an interaction between two or more polymer molecules. The molecular weight increase is particularly noticeable when certain of the ruthenium catalysts are used and the interaction between polymer molecules can be such that the hydrogenated polymer contains gelled (crosslinked) or insoluble polymer. Although a slight increase in molecular weight can be tolerated, if the molecular weight of the hydrogenated polymer is too high this causes it to be of low acceptability to the purchaser who uses it to manufacture products, such as hoses, gaskets, belts, etc.
Accordingly, the present invention is directed to an improved process for the hydrogenation of nitrile-type polymers wherein the molecular weight increase in the hydrogenation process is minimized and controlled.