Syndiotactic polymer molecules

Liquori, A. M., G. Anzunio, V. M. Cairo, M. D Alagni, P. de Santis, and M. Savina Complementary Stcreospceific Interaction Between Isotactic and Syndiotactic Polymer Molecules. Nature [London] 206, 358 (1965). 

Note that in the isotactic polymer molecule (the adapted Fischer projection shows tertiary carbon atoms of the same configuration) the successive phenyl substituents appear alternately in front of and behind the plane of the extended polymer backbone in the flat zigzag projection. However, in the syndiotactic polymer molecule (the adapted Fischer projection shows neighbouring tertiary carbon atoms of opposite configurations to each other) the successive phenyl substituents appear in front of or behind the plane of the extended polymer backbone in the flat zigzag projection. Such spatial placements of phenyl... 

The presence of a third transition between the Tg and Tm in the case of the isotactic poly(methyl chloroacrylate) has been commented on previously and evidence has been presented for the assignment of this phenomenon to the glass transition of a stereocomplex of isotactic and syndiotactic polymer molecules (3). It is not possible to confirm or reject this hypothesis on the basis of available evidence. 

Fig Planar zigzag structure of polymer molecules showing (I) isotactic, (II) syndiotactic, and (III) heterotactic configurations, (Hydrogen atoms are not shown for the purpose of clarity.)... 

In order to present clear concepts it is necessary that idealized definitions be adopted but it is recognized that the realities of polymer science must be faced. Deviations from ideality arise with polymers at both molecular and bulk levels in ways that have no parallel with the ordinary small molecules of organic or inorganic chemistry. Although such deviations are not explicitly taken into account in the definitions below, the nomenelature recommended can usefully be applied to the predominant structural features of real polymer molecules, if necessary with self-explanatory, if imprecise, qualifications such as almost completely isotactic or highly syndiotactic . Although such expressions lack the rigour beloved by the purist, every experienced polymer scientist knows that communication in this discipline is impossible without them. 

An isotactic polymer has only one species of configurational base unit in a single sequential arrangement and a syndiotactic polymer shows an alternation of configurational base units that are enantiomeric, whereas in an atactic polymer the molecules have equal numbers of the possible configurational base units in a random sequence distribution. This can be generalized as follows in zig-zag and horizontal Fischer projections ... 

There are two types of Cs-symmetric metallocenes, XXX and XXXI (Table 8-5). Both types contain a mirror plane of symmetry—a horizontal plane in XXX, a vertical plane in XXXI. Both are achiral molecules, but they differ very significantly in stereoselectivity. XXX produces atactic polymer, while XXXI usually forms syndiotactic polymer. 

It is worth noting that the absolute configuration of the tertiary carbon atoms in isotactic or syndiotactic polypropylene molecules has, in principle, no practical significance for the polymer properties, which might result from the presence of these carbon atoms in the main chain of the macromolecule. 

Two examples clearly illustrate the relationship between molecular structures of the metallocene catalysts on the one hand, and the tacticity of the resultant polymers on the other. As shown in Fig. 6.9, complexes 6.32, 6.33, and 6.34 have very similar structures. In 6.33 and 6.34 the cyclopentadiene ring of 6.32 has been substituted with a methyl and a f-butyl group, respectively. The effect of this substitution on the tacticity of the polypropylene is remarkable. As already mentioned, 6.32, which has Cs symmetry, gives a syndiotactic polymer. In 6.33 the symmetry is lost and the chirality of the catalyst is reflected in the hemi-isotacticity of the polymer, where every alternate methyl has a random orientation. In other words, the insertion of every alternate propylene molecule is stereospecific and has an isotactic relationship. In 6.34 the more bulky t-butyl group ensures that every propylene molecule inserts in a stereospecific manner and the resultant polymer is fully isotactic. 

Propene and the higher 1-alkenes can be polymerized to chains with the required degree of tacticity from almost atactic up to very highly tactic structures. However, a syndiotactic polymer can only be obtained from propene, mostly on soluble catalysts. The main factors determining controlled tactic addition are complexation, cis or trans addition, and primary or secondary addition. Most authors agree on the point that the interaction of the alkene molecule with the transition metal atom of the active centre leads to complex formation immediately before monomer insertion into the metal—polymer bond. The assumed existence of the complex is based on indirect experimental evidence and on theoretical considerations. 

The various regular polymers that can be produced by polymerization of butadiene and isoprene are summarized in reactions (4-3) and (4-4). In addition to the structures shown in these reactions, it should be remembered that 1, 4 polymerization can incorporate the monomer with cis or trans geometry at the double bond and that the carbon atom that carries the vinyl substituent is chiral in 1,2 and 3,4 polymers. It is therefore possible to have isotactic or syndiotactic polybutadiene or polyisoprene in the latter cases. Further, these various monomer residues can alt appear in the same polymer molecule in regular or random sequence. It is remarkable that all these conceivable polymers can be synthesized with the use of suitable catalysts comprising transition metal compounds and appropriate ligands. 

Polymer molecules may be linear or branched, and separate linear or branched chains may be joined by crosslinks. Extensive crosslinking leads to a three-dimensional and often insoluble polymer network. Polymers in which all the monomeric units are identical are referred to as homopolymers those formed from more than one monomer type are called copolymers. Various arrangements of the monomers A and B in the copolymer molecules (Fig. 8.1) can be produced with consequent effects on the physical properties of the resulting polymer. Synthetic polymers may have their main chains substituted in different ways, depending on the conditions of the reaction, such that atactic (random), isotactic or syndiotactic forms are produced, as diagrammatically represented in Fig. 8.1. 

Since the (4R,6R) tetramer and (4S,6S) tetramer are enantiomers, they are not separated by conventional chromatographic columns. The same is true for the (4S,6R) and (4R,6S) stereoisomers. Based on the formation of these stereoisomers, analytical pyrolysis of polypropylene is able to differentiate between isotactic and syndiotactic polymers. One fragment molecule that can be used for this purpose is the tetramer. However, other fragments from the two polymers also are diastereoisomers and, for this reason, the pyrograms of isotactic and syndiotactic polypropylene show differences. 

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