DFT calculations are reported for all possible insertions of ethene and propene in rac-Me-2-SiInd(2)Zr-R](+) and Me2C(Cp)(Flu)Zr-R](+) (R = Et and iPr). The results confirm the basic stereoregulation mechanism of Corradini. In addition, they provide an ordering of the possible sources of stereoerrors: chain misorientation is the main mechanism for unbranched (Et, nPr) chains, whereas for the beta-branched iBu chain errors due to chain misorientation and chain-olefin syn orientation are equally likely.

Chemo-, regio- and enantioselectivities in the (co-)polymerization of propene and ethene have been measured (in some cases for the first time) for a number of prototypical metallocene catalysts (in combination with methylalumoxane), and compared with those calculated at the DFT level for gas-phase cationic systems. In this paper, the first part of the study, we discuss the achiral catalysts Cp2TiCl2, Cp2ZrCl2, and Me2SiCp2ZrCl2. All three catalysts were confirmed to be highly regioselective for propene, in favor of 1,2-insertion. The performance of the titanocene in this respect is truly remarkable (only two misinsertions in 10 000 at -15 degreesC), whereas that of the two zirconocenes turned out to be worse than generally assumed (two to three misinsertions in 1000), and-quite unexpectedly-nearly identical (both in experiment and calculations), despite the decidedly more open structure of the Si-bridged system. Understandably, in all cases ethene was found to insert faster than propene, but the difference in relative reactivity of the two monomers is particularly dramatic when the growing chain is secondary, which confirms the "dormant" character of the latter for propene. The fair correlation between observed and calculated selectivities suggests that, at least for the systems considered here, solvent and counterion effects, though undoubtedly important, are rather indiscriminate and that olefin insertion is indeed the rate-determining step.