Stereospecific Polymerization of Propylene: An Outlook 25 Years after Its Discovery

Abstract

In 1955 Natta and his co-workers from the Istituto di Chimica Industriale del Politecnico di Milano reported the properties of highly crystalline polypropylene and other poly-a-olefins['l which possess, at least in long sections of the main chain, asymmetric carbon atoms of the same absolute configuration (isotactic poly-a-olefins). The discovery of the new crystalline polymers was judged at that time "revolutionary in its significance" ['] and heralded a new era in polymer science and technology. Indeed, crystalline polypropylene, because of its relatively high melting point and its outstanding mechanical properties, has found very extensive application in the field of plastics films and fibers.-25 years after this publication, it is worthwhile examining the further developments following this discovery which has had such a great scientific and industrial impact, and the problems which are still open concerning the structure of the catalytic centers and the mechanism by which practically completely stereoregular polypropylenes are formed. Historical BackgroundThe absence of crystallinity in vinyl polymers synthesized by radical polymerizations was first pointed out by Staudinger who attributed this phenomenon to the formation of a large number of diastereoisomers of the linear macromolec u l e~ [~] .Some attempts to synthesize stereoregular macromolecules were first camed out by Marvel and co-worker~ [~] who could not detect any differences in the poly(methy1 methacrylates) synthesized using optically active or nonchiral radical initiators. It was Schildknecht and his co-worker~[~I who first obtained two different types of poly(viny1 ethers) by cationic polymerization under different conditions, one of which showed some crystallinity. Unfortunately, they apparently were unaware of a paper published by Bunn ['] in 1942 who had postulated the possibility that the main chains of vinyl polymers having sterically equivalent monomeric units can assume helical conformations. Therefore Schifdknecht and co-workers, in searching for possible structures of a stereoregular vinyl polymer (Scheme l), attributed a zig-zag planar conformation like the one already known for linear paraffins, to the main chain of the poly(viny1 ethers) in the crystalline state. That is why they were compelled to attribute alternate (R) and (S) absolute configurations to the asymmetric carbon atoms of the main chain in order to eliminate strong steric interactions between the relatively bulky side chains.In 1950 Mortod8] used his "alfin" catalyst to prepare a high molecular weight polystyrene but he did not investigate its structure. In 1956, after isotactic polystyrene had been obtained and its structure identified by Natta and co-workers''], Williams ['' showed that the polystyrene obtained by Morton was isotactic.The work of Natta and co-workers in the field of poly-aolefins began in 1952, as a consequence of a report by [*) Prof. DT P. Pino, &PI.-Chem. R. Miilhaupt ETH-Zurich. Technisch-Chemisches Laboratorium Universitatstrasse 6, [**I...

Agostic interactions in transition metal compounds

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A New Synthetic Method and Solution Equilibria for the Chlorotitanate(IV) Anions − Evidence for the Existence of a New Species: [Ti2Cl11]3−

“…A shoulder on the low-frequency side of the 368 cm Ϫ1 band signals the possible presence of minor amounts of [Ti 2 Cl 11 ] 3Ϫ , which could be related to an imprecise stoichiometry during the synthesis, or to a chloride-redistribution equilibrium as shown in Equation (8). In the latter case, the equilibrium must necessarily be shifted almost completely towards the [Ti 2 Cl 10 ] 2Ϫ ion.…”

MgCl2-supported catalysts for the propylene polymerization: effects of triethers as internal donors on the activity and stereoselectivity

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