Mechanistic investigations of zirconocene alkene polymerization catalysts: Reactivity with alpha-olefins and dihydrogen

Zirconocene-catalyzed alpha-olefin polymerization is purely a kinetic phenomenon; the relative rates of monomer insertion (which includes insertion into a Zr-Me bond, insertion into a Zr-alkyl bond, 1,2-alkene insertion, and 2,1-alkene insertion), isomerization, beta-hydride elimination and hydrogenolysis affect the activity of the catalyst and the properties of the polymer, such as molecular weight and tacticity. We report the direct observation of zirconocene catalyst speciation and the rates of monomer insertion and hydrogenolysis by low-temperature 1H NMR spectroscopy for the [rac -(C2H4(1-indenyl)2)ZrMe][MeB(C 6F5)3] system. As expected, the rate of alkene insertion into a Zr-alkyl (alkyl ≠ Me) bond is faster than insertion into a Zr-Me bond. The rates of hydrogenolysis for several Zr-polyalkenyl species have been measured. The rate of alkene insertion and hydrogenolysis for a model secondary Zr-alkyl compound do not indicate that secondary Zr-alkyls are dormant or inactive species for this catalytic system.; The choice of activator for has a profound effect on catalyst activity and polymer properties. However, many less-coordinating anions are difficult to study by direct observation methods. The use of more polar solvent mixtures can mimic the effects of a more weakly bound counterion. Rates of initiation, propagation, and termination have been measured for 1-hexene polymerization catalyzed by [rac-(C2H4(1-indenyl) 2)ZrMe] [MeB(C6F5)3] in toluene/chlorobenzene solvent mixtures using both direct observation by 1H NMR spectroscopy and quench-flow methods. While initiation and propagation rates increase exponentially with increasing solvent polarity, termination rates increase linearly with increasing solvent polarity.