The strange case of the "oscillating" 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.