Isotactic structure

The polymers compared all have similar crystal structures but are different from polyethylene, which excludes the possibility for also including the latter in this series. Also note that the isotactic structure of these molecules permits crystallinity in the first place. With less regular microstructure, crystallization would not occur at all. 

The regular syndiotactic and isotactic structures are capable of crystallisation whereas the atactic polymer carmot normally do so. In the case of polypropylene the isotactic material is a crystalline fibre-forming material. It is also an important thermoplastic which can withstand boiling water for prolonged periods. Atactic polypropylene is a dead amorphous material. Polystyrene as commonly encountered is atactic and glass-like but the syndiotactic material... 

An interesting aspect of the benzofuran cationic polymerization was uncovered by Natta, Farina, Peraldo and Bressan who reported in 196160,61 that an asymmetric synthesis of an optically active poly(benzofuran) could be achieved by using AlCl2Et coupled with (-)j3-phenylalanine, (+)camphorsulphonic acid or with (-)brucine. The optical activity was definitely due to the asymmetric carbon atoms in the polymer chain, indicating that at least some of the polymer s macromolecules possessed a di-isotactic structure, v/ z.62 ... 

The elements of mirror symmetry d, m, and c can be removed in different ways, resulting in different classes of chiral polymers. Plane d containing the polymer chain is eliminated by the presence, in the main chain, of tertiary carbon atoms —CHR—), or of quaternary atoms with different substituents (—CR R"—), or with equal chiral substituents (—CR R —). Mirror glide plane c does not exist in isotactic structures, nor in syndiotactic ones in which the substituents are chiral and of the same configuration, 75 (33, 263). The perpendicular planes, m, are eliminated by the presence of chiral substituents of the same sign in syndiotactic, 75 (33, 263) or isotactic structures, 76 (263) or if the two directions of the chain are rendered nonreflective. This last condition can be realized in different ways some of which follow (264) ... 

The third structure consists of mixtures of the syndiotactic and isotactic structures favoring neither structure. This mixture of structures is called the aPP, the a meaning having nothing to do with. Space-filling models of all three different tactic forms are given in Figure 5.5. 

The sp3 stereocenter (i.e., C ) in XII is chirotopic, like the case of poly(propylene oxide), since the first couple of atoms of the two chain segments are considerably different. The isotactic structures are optically active while the syndiotactic structures are not optically active. 

The first reported instance of stereoselective polymerization was probably the cationic polymerization of isobutyl vinyl ether in 1947 [Schildknecht et al., 1947]. A semicrystalline polymer was obtained when the reaction was carried out at —80 to —60°C using boron tri-fluoride etherate as the initiator with propane as the solvent. The full significance of the polymerization was not realized at the time as the crystallinity was attributed to a syndiotactic structure. X-Ray diffraction in 1956 indicated that the polymer was isotactic [Natta et al., 1956a,b], (NMR would have easily detected the isotactic structure, but NMR was not a routine tool in 1947.)... 

Polymerization of MM in the presence of syndiotactic PMM was also reported. In this case, PMM of mostly isotactic structure was produced. Interaction between growing chains and the template is rather weak. The template influence is not as pronounced as in the case of isotactic template. Matsuzaki at al used deuterated syndiotactic PMM as template for polymerization of MM. The measurements of tacticity by NMR were... 

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. 

Braun, Herner, Johnsen and Kern (19) have also studied the polymerization of methylmethacrylate by alkylmetals. The amount of isotactic structure decreased in the series alkyllithium > alkyl-sodium > alkylpotassium. The use of polar solvents also decreased the amount of isotactic structure (see Table 3). 

The polymerization of butadiene to 1.2 polymers with anionic Ziegler type catalysts has been studied by Natta and co-workers (46). They have shown that isotactic 1.2-polybutadiene can be produced by the use of catalysts which are made up of components which have basic oxygen and nitrogen structures such as triethylaluminum with cobalt acetylacetonate or with chromium acetylacetonate. Natta and co-workers have shown that either syndiotactic or isotactic structures are produced depending on the ratio of aluminum to chromium. Syndiotactic structures are obtained at low aluminum to chromium ratios while isotactic polybutadiene is obtained at high ratios. The basic catalyst component is characteristic of syndiotactic catalysts. Natta, Porri, Zanini and Fiore (47) have also produced 1.2 polybutadiene using... 

The mechanism of the polymerization of propylene to produce isotactic structure has been studied extensively. Natta and his coworkers hav presented the generally accepted anionic-coordinate mechanism. However, there has been increasing evidence that the Natta anionic-coordinate mechanism does not operate to produce isotactic polypropylene. 

However, Natta, Pasquon, Zambelli and Gatti (63) have shown that titanium trichloride-dialkylaluminum chloride or titanium trichloride and alkylaluminum dichloride with nucleophilic materials are good catalysts for the polymerization of propylene to the isotactic structure. The titanium trichloride could also be made in situ from titanium tetrachloride and triethyl aluminum. Vesely, Ambroz, Vilin and Hamrik (64) showed the addition of the nucleophilic materials to diethylaluminumchloride-titanium trichloride polymerizations decreased the rate of polymerization and changed the stereospecificity. The more nucleophilic materials such as sulfur compounds were more effective than the less nucleophilic oxygen materials. 

A wide variety of homogeneous and hetrogeneous systems are effective for polymerizing various monomers to isotactic structures. Therefore the question must be raised as to whether macro-surface theories have any required validity for steric control but whether some other factor is important. Fig. 10 summarizes the monomers polymerized to isotactic polymers and shows that all propagate with an asymmetric center at the end of the chain. 

The same concept is presented by Tsuruta, Inoue, Ishimori and Yoshida (95) in their study of propylene oxide polymerization. They concluded that the stereochemistry of the monomeric units in the isotactic structure is regulated by the asymmetric center of the proceeding monomer unit. 

The electron releasing or attracting substituents on the carbon-carbon double bond determines the balance of ionic nature required of the gegen ion of the catalysts to produce isotactic structure. 

In all of the above discussions we have treated only the monomers with carbon-carbon double-bonds. It is probable that polymers of non-oleftnic monomers such as polypropylene oxide (103, 104, 105) poly-ethylidene (106, 107, 108, 109) and polyaldehydes (110, 111) polymerize to isotactic structures by the same mechanism. The same correlation of ionicity of the catalysts with the isotactic structures and syndiotactic structures should also be possible. 

The polymers of the optically active and racemic 4-methyl-1-hexene and the poly-(S)-5-methyl-l-heptene have isotactic structure (115) the same structure seems probable also in the case of the other polymers prepared till now from optically active or racemic a-olefins. 

The polymers have isotactic structure with helix conformations in the solid state (78) polymers of optically active and racemic (1-methyl-propyl)-vinyl-ether seem to have the same crystalline structure (Table 12). 

Any of the four monomer residues can be arranged in a polymer chain in either head-to-head, head-to-tail, or tail-to-tail configurations. Each of the two head-to-tail vinyl forms can exist as syndiotactic or isotactic structures because of the presence of an asymmetric carbon atom (marked with an asterisk) in the monomer unit. Of course, the random mix of syndiotactic and isotactic, ie, atactic structures also exists. Of these possible structures, only... 

Using such a-olefins as monomers, Natta (4,5) found that most of the product had a stereoregular structure. Thus, under conditions easy to realize, isotactic structures (A)... 

Propylene content in EPM rubber can be determined with the help of IR spectra. A propylene band near 1155 cm 1 has been widely used [79] for EPM analysis, frequently in combination with the polyethylene band at 721 cm"1. Tacticity is important in EPM rubber, and the bands at 1229 and 1252 cm"1 are characteristic of syndiotactic and isotactic structures, respectively, (both bands are present in atactic polypropylene as well). Polymer structure may vary in the relative tactic placement of adjacent head to tail propylene units and in the sequence distribution of base units along the chain. Some of them can be identified [80] by infrared spectra, such as isolated or head to tail propylene units ... 

Palladium-based catalysts bearing chiral ligands have also been found to be capable of the stereospecific copolymerisation of allylbenzene [492] and its derivatives [493] with carbon monoxide the formed copolymers appeared to be of an alternating, isotactic structure. 

Subsequent investigations revealed that, in principle, styrene undergoes isospecific polymerisation in the presence of heterogeneous Ziegler Natta catalysts [1-4], Although polystyrene of isotactic structure was also prepared with the use of homogeneous nickel-based coordination catalysts, it appeared to be of low molecular weight [22,23]. 

---