Alkene Polymerization Ti, Zr, Sc, and others

The catalytic cycle for alkene polymerization, the Cossee mechanism, is extremely simple. Coordination of the alkene to Cp2ZrR+ is followed by insertion to give a new complex Cp2ZrR+. No change in the oxidation state of Zr occurs in either of these steps. 

There are several mechanisms for termination of the growing polymer chain. One common chain termination step is jS-hydride elimination to give Cp2ZrH+ and the polymer with a terminal double bond. When the polymerization is carried out under H2, a cr-bond metathesis can take place to give the saturated polymer and Cp2ZrH +.  

When alkenes higher than ethylene are polymerized, regio- and stereochemical issues arise. The regiochemical issue arises when the alkene H2C=CHR in- 

Problem. 6.5. When alkenes are subjected to polymerization conditions in the presence of a sufficiently high concentration of H2 gas, alkene hydrogenation occurs instead of polymerization. Draw a reasonable mechanism for this transformation. 

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