Natta-Corradini structure

The gelation of iPS has already been discussed in terms of a phase separation, but there is also strong evidence intimately linking crystallization and gelation. Indeed, the crystallographic structure of the micellar gel crystals is distinctly different from the traditional Natta-Corradini structure and appears to be unique to gels. This new structure has a fibre-repeat close to a fully extended (al -trans) conformation but is actually a 12i helix, in contrast to the 3i-helix of... 

G. Natta and P. Corradini. Structure and properties of isotactic polypropylene , Nuovo Cimento, Suppl. 15, 40 (1960). 

At the present time, the most likely concept of the mechanism of a heterogeneous polymerization catalyzed by a Ziegler-Natta catalyst involves a complex in which the organometallic component and the transition metal component—i.e., the A1 and Ti atoms—are joined by electron-deficient bonds. Natta, Corradini, and Bassi (13) have reported such a structure for the active catalyst prepared from bis (cyclopentadienyl) titanium dichloride and aluminum triethyl. Natta and Pasquon (14), Patat and Sinn (18), and Furukawa and Tsuruta (2) have proposed mechanisms for the stereospecific polymerization of a-olefins in terms of such electron-deficient complexes. 

Mesophase reveals intermediate order between amorphous and crystaUine phases. In the first studies it was labelled as smectic (Natta Corradini, 1960) or paracrystalline (Miller, 1960). Further studies revealed that mesophase is made up of bundles of parallel chains, which maintain typical for all polymorphic forms of polypropylene three-fold helical conformation. Bundles are terminated in the direction of the chain axis by helix reversals or other conformational defects (Androsch et al., 2010). In the bundles long range ordering maintains only along the chain axes, whereas in lateral packing a large amount of disorder is present (Natta Corradini, 1960). The mesophase is formed by quenching of the molten polypropylene (Miller, 1960 Wyckoff, 1962) or by deformation of the crystalline structure (Saraf Porter, 1988 Qiu, 2007). As for the fibres, the mesophase was observed in fibres taken at low take-up velocity (Spruiell White, 1975 Jinan et al., 1989, Bond Spruiell, 2001) in fibres intensively cooled in water with addition of ice or in the mixture of dry ice... 

The structure of the a form was early established. Natta and Corradini (Natta Corradini, 1960) proposed a model based on the monoclinic unit cell. According to this model, polypropylene helices of the same hand are arranged in layers parallel to the ac plane. Layers of isochiral helices alternate with layers formed from helices of the opposite hand. The model was commonly accepted and confirmed by subsequent studies of Menok (Mencik, 1972), Hikosaka (Hikosaka Seto, 1973 ) and Corradini (Corradini et al., 1980). 

The butadiene polymers represent another cornerstone of macromolecular stereochemistry. Butadiene gives rise to four different types of stereoregular polymers two with 1,2 linkage and two with 1,4. The first two, isotactic (62) and syndiotactic (25), conform to the definitions given for vinyl polymers, while the latter have, for eveiy monomer unit, a disubstituted double bond that can exist in the two different, cis and trans, configurations (these terms are defined with reference to the polymer chain). If the monomer units all have the same cis or trans configuration the polymers are called cis- or trans-tactic (30 and 31). The first examples of these stereoisomers were cited in the patent literature as early as 1955-1956 (63). Structural and mechanistic studies in the field have been made by Natta, Porri, Corradini, and associates (65-68). 

G. Natta, P. Corradini, I. W. Bassi. Crystal structure of isotactic polystyrene ,... 

A variety of structures have been proposed for the active species in Ziegler-Natta initiator systems [Allegra, 1971 Arlman and Cossee, 1964 Corradini et al., 1989 Natta, 1960a,b Patat and Sinn, 1958 Rodriguez and van Looy, 1966 Tait and Watkins, 1989]. Structure... 

The polymerization of vinylethers to produce isotactic structures has been one of the oldest examples of stereoregular polymerization. This topic has been well reviewed by Natta, Dall Asta, Mazzanti, Giannini and Cesca (6). Natta, Bassi, Corradini (7) showed that the polyvinylethers first studied by Schildknecht were indeed isotactic polymers. 

Fig. 10. Structure and symmetry elements of crystalline polyvinyl chloride [Natta and Corradini (154)]...

On the basis of x-ray and infrared studies we may now accept the syndiotactic chain configuration for crystalline PVC. If, in addition, we accept the proposed crystal structure [Natta and Corradini (154)), it is possible to base the symmetry analysis of the spectrum on the unit cell instead of just on a single chain, as was done in the earlier work [Krimm and Liang (101)). The results of this analysis are given here. The unit cell, shown in Fig. 10, has the following symmetiy elements E, Cs2(a), 6(b). C (c), [Pg.123]

The structure of isotactic polypropylene has been investigated by x-ray diffraction [Natta and Corradini 151) Natta, Cor-radini, and Cesari 155) Natta and... 

The structure of isotactic polystyrene has been studied by means of x-ray diffraction [Natta and Corradini (152,153)]. It is found that the chain configuration is helical, with three monomer units per repeat exactly as in polypropylene. The only difference is that the chain repeat of 6.65 A in polystyrene implies an opening of the CCC bond angles in the chain to 116.5°. The orientation of the benzene ring is apparently not settled. In the above structure, if the axis of the benzene ring were perpendicular to the helix axis, then the plane of the benzene ring would be almost exactly perpendicular to the helix axis. In a variation of this structure proposed on the basis of stereochemical considerations [Bunn and Howells (30)], and shown in Fig. 16, the plane of the benzene ring... 

For this isotactic polypropylene, different structures based on a 3i helix chain structure have been proposed. Natta and Corradini (16) reported that the space group is Cc or C2/c,... 

An isotactic structure is one in which the optically active centers of the repeat units all have the same absolute stereochemistry (G. Natta, P. Pino, P. Corradini, F. Danusso, E. Mantica, G. Mazzanti, G. Moraglio, J. Am. Chem. Soc. 1955, 77, 1708) in a syndiotactic polymer, neighboring units have opposite stereochemistry. If an isotactic polyolefin is drawn in its extended conformation, it will have all its substituents pointing in the same direction. If an isotactic polyketone is drawn in its extended conformation, the substituents will alternately point up and down. 

Natta G, Corradini P (1960) The crystal structure of cis-1,4-polybutadiene. Nuovo Cimento, Suppl. 15 111 see also Angew. Chemie 68 615 (1956) and Nyburg SC (1954) Acta Cryst. 7 385... 

Figure 3.12 Packing in the crystai structure of poiyethyiene as viewed aiong the c-axis. (From Natta, G. and Corradini, R, Rubber Chem. Techno ., 33, 703, 1960. With permission.)... 

Albeit indirectly and ex-post, I was personally involved in the story as a coworker of Paolo Corradini and Adolfo Zambefii, the two associates of Natta who elucidated, respectively, the crystal structure and the microstracture of polypropylene. Moreover, a large part of my professional life (more than three decades, alas) has been dedicated to the stereochemistry of this fascinating polymer. I decided to let all this transpire from the pages, which may imply here and there a subjective opinion but no deliberate bias. 

The team in Milan had to face two major scientific problems assign the structure of the new polymer and explain its genesis. The first part could be accomplished with a clever analysis of the X-ray diffraction patterns, recognized by Corradini [20 and references therein] as compatible with the 3i helical conformation anticipated by Bunn [21] for a hypothetical stereoregular hydrocarbon with the configuration that we now name isotactic after the suggestion by Natta s wife [11] (Fig. 3). 1 am not a believer in Lukacs reflection theory [22], but 1 find it a fascinating coincidence that Watson and Crick published their fundamental work on the a-helix of DNA at about the same time [23]. 

Natta, G. and Corradini, P. (1960) Structure and properties of isotactic polypropylene, Nuovo Cimento Suppl., 15,40-51. 

Natta, G Corradini, R Cesari, M. Crystalline structure of isotactic polypropylene. Atti Accad. Nad-Lincei, Rend., Classe Sci. Fis., Mat. e Nat. 1956, 21, 365-372 Chem. Abstr. 1957,51, 65132. 

Natta, G Porri, L. Corradini, P. Morero, D. Stereospecific polymerization of conjugated diolefins. 1. Synthesis and structure of compounds with l,A-trans linkage. Chim. Ind. (Milan) 1958, 40, 362-371 Chem. Abstr. 1959, 52, 86086. 

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