Ziegler-Natta catalysis: Forever young
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Introduction Introducing a book or a series of articles dedicated to Ziegler-Natta chemistry in 2013 cannot elude two questions predictably raised in the introductory article: How far is this 60-year-old catalysis field from maturity? And does it make sense to insist on fundamental and exploratory research on the subject? As an academic scientist in the area since the early 1980s, I long ago felt the need to answer the very same questions for myself when, at my midlife crisis, I faced the classical dilemma: “Shall I stay or shall I go?” My conclusion was, and still is, that the problem of maturity at some point would rather be my own, because olefin polymerization belongs among those combinatorial human creations that do not age. To draw a parallel with art, writers and music composers may spend their whole lives assembling letters and notes into original combinations, and literature and music will continue to represent a source of novelty and surprise. In a similar way, with their unique ability for precise control over the catalytic process, polyolefin chemists know how to enchain olefin monomers into ever new molecular architectures,1,2 and as long as they exist, their chemistry will stay young.
Polypropylene-based materials: A paradigmatic story The case of isotactic polypropylene (iPP)1–3 is well suited to illustrate the concept. Let us consider what can be achieved
with just three basic constitutive elements (namely the m (meso) and r (racemo) diads in regioregular strings of propylene units,2 and some ethylene units (E) copolymerized with the former;3 Figure 1). Very recently, papers have appeared claiming the most highly stereoregular homo-iPP ever: Kol and co-workers used novel post-metallocene Ti-based complexes with “Salalen” ligands and an intriguing C1-symmetry;4 Rieger and co-workers replied with more classical C2-symmetric bis(indenyl) ansazirconocenes and hafnocenes featuring a properly designed (albeit pretty complex) substitution pattern.5 Their perfect “ . . . mmmmmmmmm . . . ” strings are certainly impressive, and possibly associated with remarkable properties, but they only represent one vertex of a vast triangle with infi nite possibilities. We may want to step into it by randomly introducing just a few Es in the long m strands; in the resulting iPP “random copolymers,”3,6 the ethylene units perturb the crystallization of the propylene homosequences, and practically all properties of the material (physicomechanical parameters, transparency, sealability, gas permeability, feeling at touch, and many more) change in a highly controlled and tunable fashion (provided that our catalyst is good enough). A relevant application can be seen by anyone who has flown economy class on airlines. Until recently, when offered a drink, we used to receive perfectly transparent but very brittle atactic polystyrene cups; storing one such empty cup “in the back-pocket of the seat in front of us” and hearing the “crack” was practically one
Vincenzo Busico, Dipartimento di Scienze Chimiche, Università di Napoli Federico
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