| Abstract: | Polyethylene exhibits an increase in melt viscosity (melt index drop-off) when subjected to extreme thermal treatment. Although stabilizers and antioxidants do not prevent this change it has been possible to stabilize high-density polyethylene by isomerization or hydrogenation of the polymer unsaturation. A soluble nickel octoate–triethylauminum complex was an active catalyst system for these reactions. A triethyl-aluminum/nickel ratio of three appeared optimum for maximum activity. Of particular interest regarding this catalyst system was the fact that the terminal vinyl unsaturation of the polymer had to be isomerized to some internal position before hydrogenation commenced. However, when this catalyst system was impregnated on a predried silica or when butyllithium was substituted for triethylaluminum, both isomerization and hydrogenation occurred simultaneously. The addition of a light olefin such as ethylene to the reaction mixture slowed the overall reaction until most of the ethylene had been hydrogenated. At this point the isomerization–hydrogenation reaction path previously mentioned commenced. |
