Elemental Reactions in Copolymerization of α-Olefins by Bis(cyclopentadienyl) Zirconocene and Hafnocene: Effects of the Metal as a Function of the Monomer and the Chain End

The effect of the metal on elementary chain propagation and termination reactions in hafnocene- and zirconocene-catalyzed olefin copolymerization processes has been systematically studied by quantum chemical methods. Two consecutive monomer insertions, parallel with the competing chain termination reactions, were studied for copolymerization of ethene with propene, 1-butene, and 1-hexene. For the purpose of a comparative study, analogous species along the reaction pathway were studied for each metallocene/monomer combination. Effects due to incorporation of a comonomer were analyzed as a function of both the central metal and the comonomer size. In accordance with the general experimental observations of zirconocenes producing lower molecular weight polymer than the hafnocenes, differences in activation energies for chain propagation and termination are smaller for the zirconocene. For the zirconocene, the activation energy is particularly low for beta-hydrogen elimination after secondary comonomer insertion.