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Based on the recent literature, the synthesis of novel polyolefin architectures via Ziegler-Natta chemistry under living conditions appears to be a promising and fast-growing area. In this paper, the appropriateness of the term living as applied to the said context is discussed in mechanistic terms, and the potential and the limitations of the field are outlined with the aid of two paradigmatic case histories (i.e., propene polymerization in the presence of bis(phenoxy-imine)Ti and bis(phenoxy-amine)Zr catalysts).

The field of stereoselective propene polymerization has been dramatically innovated by the discovery of homogeneous metallocene-based catalysts with well-defined and tunable molecular structure. Of all, "oscillating" metallocenes are probably the most ingenious and challenging example of catalyst design. Their catalytic species were built to "flip-flop" between a chiral and an achiral conformation, at a rate intermediate between those of monomer insertion and chain transfer. The result of this molecular switching would be a polypropylene with an isotactic/atactic stereoblock structure, performing as a thermoplastic elastomer. This essay discusses how the real polymerization mechanism differs from what the catalyst inventors had in mind, but also how - through fortunate circumstances their optimism has been rewarded.

In this presentation, some mechanistic aspects of isotactic propene polymerization in the presence of homogeneous Group IV metallocene catalysts are analyzed in the light of new (and still largely unpublished) experimental results. In particular, the regiospecificity of catalyst systems based on two typical C2-symmetric ansazirconocenes (i.e. rac-ethylenebis(1-indenyl) ZrCl2 and rac-dimethylsilylbis(1-indenyl) ZrCl2; co-catalyst, methylalumoxane) is discussed comparatively. An influence of propene concentration on the ratio between 2,1 and 1,3 monomer enchainments and surprisingly - on the stereoregularity of the produced polymers is pointed out. Tentative kinetic explanations of the observed polymerization behaviours are also proposed.