Monofunctional and difunctional-terminated liquid homopolymers derived from butadiene and isoprene monomers are suitable for a variety of applications in the polymer industry, and are particularly desirable due to their relatively low cost. However, the presence of unsaturation in the aliphatic hydrocarbon backbones of these monomers render the polymers produced therefrom thermooxidatively unstable. That is, such polymers are susceptible to breakdown through oxidation catalyzed by heat. However, by hydrogenating the unsaturation present in these polymers to a high degree, the thermooxidative stability of the polymers can be substantially improved. More particularly, removal of at least about 95 percent of the unsaturation in monofunctional and difunctional-terminated polybutadiene or polyisoprene significantly improves their thermooxidative stability, and saturation of at least about 99 percent of the backbone double bonds results in a polymer having superior thermooxidative stability.
Catalytic hydrogenation is a conventional way of obtaining saturated materials from monofunctional and difunctional-terminated conjugated diene polymers. However, cost-effective heterogeneous catalysts, such as Raney nickel, which are suitable for the hydrogenation of most monomers are, in general, unsuitable for hydrogenation of monofunctional and difunctional-terminated conjugated diene polymers to a high degree, presumably due to the inaccessibility of the polymer backbone to the catalyst. This inaccessibility is due to the coiled conformation that these polymers assume. In some instances, specially prepared heterogeneous noble metal catalysts with a high surface area, such as rhodium and palladium-based heterogeneous catalysts, have been shown to be successful in such an application, but are not cost-effective due to their inherent high cost as well as increased expense related to the special preparation of these catalysts. As alluded to above, in order for a hydrogenation catalyst to be effective, intimate contact between the catalyst and the polymer backbone is necessary. Hence, preferred catalysts for the hydrogenation of these polymers are homogeneous rather than heterogeneous, since such homogeneous catalysts achieve this intimate contact.
Rhodium and palladium-based homogeneous catalysts, and in particular tristriphenylphosphinerhodium (I) chloride, have heretofore been the catalysts of choice for hydrogenating functional-terminated conjugated diene polymers. These catalysts are chosen because they achieve intimate contact with the polymer backbone and further because they are unaffected by the functional groups of the polymers, resulting in a high degree of hydrogenation of the unsaturated polymer backbone. However, rhodium and palladium-based homogeneous catalysts are very expensive. Although more conventional homogeneous catalysts based on titanium, iron, cobalt, or nickel are generally about 1,000 times less expensive than the rhodium or palladium-based homogeneous catalysts, these conventional homogeneous catalysts are easily deactivated or "poisoned" in the presence of certain functional groups in the polymer being hydrogenated. For example, a cobalt-based homogeneous catalyst would undergo oxidative addition with the terminal hydroxy groups of a monofunctional or difunctional hydroxy-terminated polybutadiene polymer, thereby causing chain extension and subsequent gelation of a solution of the polymer, thus making the polymer unsuitable for hydrogenation. Aluminum trialkyls, which are commonly used reductants in the formation of active cobalt species, would also exert a similar effect on the polymer.
Japanese patent 62151404 relates to hydroxy-containing diene polymers which are catalytically hydrogenated in solvents, then treated with alcohols to separate highly-hydrogenated polymers from low or non-hydrogenated polymers. Thus, 300 g of OH-terminated polyisoprene was hydrogenated over Ni-diatomaceous earth in hexane at 130.degree. and 50 kg/cm.sup.2 -gage H for 4 hours, then shaken with 400 cm.sup.3 iso-PrOH and left to stand to separate into two layers, the top of which contained 103 g of 97 percent hydrogenated polymer and the bottom of which contained 200 g of 56 percent hydrogenated polymer.
Japanese Patent 62151405 relates to OH-terminated diene polymers which are catalytically hydrogenated in high yield by solution hydrogenation, treatment with alcohols, and rehydrogenation of the resulting dark solution. Thus, 300 g of OH-terminated polyisoprene was hydrogenated in n-hexane with a Ni-kieselguhr catalyst and the solution treated with 400 g of 2-propanol to form transparent (hydrogenated to 97 percent) and dark-colored (hydrogenated to 56 percent) fractions. The colored fraction was rehydrogenated to obtain a hydrogenation ratio of 67 percent.
Japanese Patent 53026890 relates to using Raney Ni catalysts, to hydrogenate OH-containing unsaturated hydrocarbon polymers in alcohols or mixtures of alcohols and other organic solvents. Thus, 50 g of polybutadiene polyol (having a number average molecular weight of 3100 and an OH value of 0.82 mequiv./g) 50 g of iso-PrOH, and 5 g of Raney Ni catalyst were hydrogenated for 6 hours at 150.degree. and 50 kg/cm.sup.2 H pressure in an autoclave to give 50 parts polymer (having 0.83 mequiv./g OH value). The hydrogenation degree was 63.8 percent, as compared with 19.5 percent when cyclohexane was used instead of iso-PrOH.