Hydrogen transfer, during olefin polymerization

Chung, T.C. Dong, J.Y. A novel consecutive chain transfer reaction to p-methylstyrene and hydrogen during metallocene-mediated olefin polymerization. J. Am. Chem. Soc. 2001,123,4871. 

Reaction 1 appears to result solely in termination. In hydrogenolysis experiments with various chelates we have observed precipitation of lithium hydride in all cases at room temperature. Attempts to generate chelated LiH in situ by adding hydrogen during ethylene polymerization also caused a rapid, irreversible loss of activity. Since there is no evidence that lithium hydride can add to ethylene under moderate polymerization conditions, it is unlikely that any significant chain transfer occurs via this mechanism. Potassium alkyls readily eliminate olefin with the formation of metal hydride, and sodium alkyls do so at elevated temperatures (56). It was noted earlier that chelation of lithium alkyls makes them more like sodium or potassium compounds, so it is quite probable that some termination occurs by eliminating LiH. It is conceivable that this could be a chain transfer mechanism with more reactive monomers than ethylene because addition to lithium hydride would be more favorable. 

The discovery of conjunct polymerization, whereby hydrogen transfer occurs between molecules of olefins, leading to the formation of saturated hydrocarbons and polyenes, became the forerunner of the discovery of alkylation and isomerization of paraffins. During World War II these processes were used for the manufacture of high octane aviation gasoline to be used in pursuit planes, and presently products from the alkylation reaction assumed new importance as a major component of unleaded gasoline (Figure 4). 

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