Mechanistic investigations in metallocene-catalyzed alkene polymerization through direct observation of the catalytic species

Metallocene catalyzed olefin polymerization has revolutionized the polymer industry, due to high stereoselectivities and activities. Rational modification of the ligand structure dramatically changes the polymer microstructure and molecular weight, allowing the formation of novel polyolefin materials. Despite the industrial importance of these catalysts, many mechanistic features of polymerization are still unclear. Direct NMR observation of the [ rac-(C₂H₄(1-indenyl)₂)ZrMe][MeB(C ₆F₅)₃] catalyst system provides a straightforward approach to determine mechanisms and kinetic rate laws of polymerization.; Trapping the active catalyst species at low temperature permits the direct observation of the reactivity at the metal center using NMR spectroscopy. By directly observing the metallocene-polyhexenyl or polypropenyl species, rates of propagation and termination have been determined.; Bulk polymerization of propene and study of the 13C NMR microstructure of the polymer provides a printout of the reactions that occur at the metal center. Comparison of microstructure data obtained with the [ rac-(C₂H₄(1-indenyl)₂)ZrMe][MeB(C ₆F₅)₃] catalyst system allows a straightforward comparison of published microstructure data using different anions.; Chain end epimerization inverts the stereochemistry of the last inserted monomer. At low [propene], this causes a dramatic lowering of the polymer tacticity. Direct observation of chain end epimerization using isotopically labeled propene provides insight into the mechanisms that occur during chain end epimerization.; Secondary zirconocene alkyls have been proposed to be catalyst dormant states because insertion of propene into the Zr-alkyl bond is assumed to be slow. Generation of a secondary-Zr alkyl in situ permits the observation of propene insertion into the secondary Zr-alkyl bond. The rate constant of propene insertion into a secondary Zr-alkyl has been determined, and the implications of this rate and the possibility of catalyst dormant sites are discussed.