Accelerated Oxidations of Polyisoprene. I. Metal-Catalyzed Oxidations in Solution
Authors: Mayo Frank R. , Egger Kurt , Irwin Katherine C. 
Published: February 2011

Abstract
Most of our work was carried out at 50° C with about 2 per cent solutions of synthetic polyisoprene in chlorobenzene. In many experiments, oxygen absorption and decrease in viscosity were followed together. Viscosity changes are a very sensitive measure of chain cleavage of high molecular weight polymers but of little use with degraded polymers; for the latter some number average molecular weights are available. The effects of several metal compounds were examined on both undegraded and partly degraded rubber solutions; for metal catalysis some peroxides are also required. Unless oxidation is initiated by a free radical source such as 2,2′-azobis-(2-methylpropionitrile) or unless a peroxide (e.g., t-butyl hydroperoxide) is added initially, considerable time elapses before the catalytic effect of the metal becomes obvious. The lower valence form of the metal often has a net retarding effect on oxidation at this stage. In partially oxidized solutions, however, the metals catalyze oxidation at once. Addition of metal salts to degraded rubber solutions, even in the absence of oxygen, causes decomposition of peroxides and cleavage of chains. The abilities of metal compounds to decompose peroxides, to cleave chains, and to catalyze oxidation differ considerably. On starting with undegraded rubber solution, and either AZBN or one of several metal compounds, the oxygen absorbed for a given reduction of viscosity was nearly constant even though the rates of oxidation varied widely. In two experiments on a rubber solution without metals (where chain cleavages could be determined more accurately), about 48 molecules of oxygen were absorbed and four chains were cleaved per initiating radical. About one cleavage occurred per 14 oxygen molecules absorbed in chain propagation, independent of the rate of chain initiation. This ratio seems to apply fairly generally. Thus chain cleavage is a primary reaction accompanying chain propagation, rather than chain termination, and added metal compounds affect mainly the rates of initiation and termination rather than chain propagation. Cobalt compounds are the most active metal catalysts for oxidation of rubber. Except for a few inactive complexes, differences among cobaltous compounds so far have been small. The cobaltous forms lose some of their activity as they are converted to cobaltic compounds. The latter in turn lose their activity entirely when they form insoluble complexes with highly degraded rubber. Several fractions of these complexes were investigated. The catalytic activity of cobaltic compounds is more affected by solvent and coordinating groups. The ability of both cobaltous and cobaltic forms to react with peroxides is critical in catalytic activity. We have been unable to account satisfactorily for all the oxygen absorbed by our rubber solutions. Part appears as titratable hydroperoxides; part is in alcohol and carbonyl groups; the balance is unaccounted for and possibly is in ether links, cyclic or dialkyl peroxides, or hydroperoxides which we were unable to titrate. Very little of any primary, simple fragmentation product of oxidation was found. Pyrolyses of oxidized rubber gave some limonene, not enough simple oxygenated compounds to tell much about its structure, and no evidence of adjacent cyclic peroxide groups.