FIRST PRINCIPLES MOLECULAR-DYNAMICS STUDY OF ZIEGLER-NATTA HETEROGENEOUS CATALYSIS

We present a first principles study of the Ziegler-Natta MgCl2-support ed polymerization of ethylene in the framework of the Car-Parrinello a pproach. In particular we investigate the titanium active sites on the (110) surface of the support focusing on Corradini's model and the po ssible alternative configurations. We find that different catalyst sit es are allowed and that the relevant energetics as well as the reactiv ity in the alkyl chain formation process strongly depend on the local geometry. During deposition of TiCl4 and complex formation, which are energetically downhill, the dynamical approach allows us to follow the reaction pathway in an unbiased way. By means of Car-Parrinello const rained molecular dynamics we then determine the free energy profiles a nd estimate activation barriers in the alkene insertion processes. Fur thermore a dynamical study of the insertion of a second ethylene molec ule offers an interesting insight into the chain growth process and th e stereochemical character of the polymer, providing a complete pictur e of the reaction mechanism.