This has been explained1· 3 in terms of a “chain migratory” polymerization mechanism, in which the incoming monomer and the growing polymer chain exchange at each insertion step between the two available enantiotopic coordination positions. 4 Occasional failures of this mechanism (“skipped insertions”) lead to the formation of stereoirregularities {... rrrrmrrrrr...)·, intuitively, the probability of skipped insertions is higher the lower is propene concentration. 1· 3

No similar effects of monomer concentration havebeen described so far for C2-symmetric isotactic-specific catalysts. 1· 5 In these catalysts, the twocoordination positions available to the monomer and the growing polymer chain are homotopic, 4 so that monomer insertion from the same enantioface is preferred at each of the two positions for a catalytic complex with a given chirality of coordination of the aromatic ligand. Quite surprisingly, we found that the isotacticityof propene polymers prepared at a given temperature (50 C) inthe presence of a number of C2-symmetric anso-zirconocenes (Table 1; co-catalyst: MAO) is also dependent on propene concentration,[C3H6](Figure 1).