Tritactic polymer

With 1,4-disubstituted dienes, more complex stereochemistry is possible in the polymers. In addition to the possibility of geometrical isomerism, there are two types of completely assymetric centre in the molecule and tritactic polymers can... 

Thirty years later, we discovered the topochemical polymerization of various 1,3-diene monomers giving a highly stereoregular polymer in the form of polymer crystals. When ethyl (Z,Z)-muconatc was photoirradiated in the crystalline state, a tritactic polymer was produced [18, 19], in contrast to the formation of an atactic polymer by conventional radical polymerization in an isotropic state. Thereafter, comprehensive investigation was carried out, for example, the design of monomers, the crystal structure analysis of monomers and polymers, and polymerization reactivity control, in order to reveal the features of the polymerization of 1,3-diene monomers [20-23], Eventually, it was revealed that the solid-state photoreaction... 

Tactic polymers are classified as isotactic, syndiotactic, or heterotactic according to how the configurational monomeric units are bonded to each other. If two defined stereoisomeric positions per configurational monomeric unit occur, then this is known as a ditactic polymer, which may, for example, be diisotactic or disyndiotactic. If the configurational monomeric unit has three defined stereoisomeric positions, then the polymer is a tritactic polymer. Polymers of higher tactic number are similarly defined. 

Ditactic polymers possess two stereoisomeric centers per constitutional monomeric unit, and tritactic polymers possess three. Ditactic polymers may be formed by the polymerization of 1,2-disubstituted ethylene derivatives, as, for example, with pentene-2 ... 

HGURE 1.13. Tritactic polymers, (a) Trans-erythw tritactic polymer, (b) trans-threo tritactic polymer, (c) cis-erythro tritactic polymer, and (d) cis-threo tritactic polymer. 

Polymerization of 1,4-disubstituted butadienes can lead to products that possess two asymmetric carbon atoms and one double bond in each repeat unit. Such tritactic polymers are named with prefixes of cis or trans together with erythro or threo (see Fig. 1.13). 

An example of an asymmetric induction from optically inactive monomers in an anionic polymerization of esters of butadiene carboxylic acids with (/ )-2-methylbutyllithium or with butyllithium complexed with (-)-menthyl ethyl ether as the catalyst. The products, tritactic polymers, exhibit small, but measurable, optical rotations. Also, when benzofiiran, that exhibits no optical activity, is polymerized by cationic catalysts like aluminum chloride complexed with an optically active cocatalyst, like phenylalanine, an optically active polymer is obtained. ... 

What are tritactic polymers Draw cis and trans tritactic polymers. 

Obviously as the complexity of the monomer increases, greater orders of tacticity are possible. To date only one form of a tritactic polymer has been isolated, and that by polsonerizing an ester of trans-trans sorbic acid (CH3CH=CHCH=CHCOOR) (Natta et al., 1960b). The polymer, which contained three elements of stereoisomerism, two optical and one geometrical, was identified as erythro-diiso-trans tactic. 

Tritactic polymer n. An isotactic or syndio-tactic polymer that is also of the cis- or trans form because the molecules are unsaturated and have double bonds. 

The combination of cis-trans isomerism with iso-syndio and erythro-threo dispositions gives complex stractures as exemplified by the 1,4 polymers of 1-or 4-monosubstituted butadienes, such as 1,3-pentadiene (72, 73), and 2,4-pentadienoic acid (74, 75) and of 1,4-disubstituted butadienes, for example, sorbic acid (76). This last example is described in 32-35 (Scheme 6, rotated Fischer projection). Due to the presence of three elements of stereoisomerism for each monomer unit (two tertiary carbons and the double bond) these polymers have been classed as tritactic. Ignoring optical antipodes, eight stereoregular 1,4 structures are possible, four cis-tactic and four trans-tactic. In each series (cis, trans) we have two diisotactic and two disyndiotactic polymers characterized by the terms erythro and threo in accordance with the preceding explanation. It should be noted that here the erythro-threo relationship refers to adjacent substituents that belong to two successive monomer units. 

Note 1 Terms referring to the tacticity of polymers (tactic, ditactic, tritactic, isotactic, cistactic, etc.) can also be applied with similar significance to chains, sequences, blocks, etc. 

Natta carried out the anionic polymerization of methyl sorbate, a 1,3-diene, with an optically active initiator and obtained an optically active homopolymer with main-chain chirality. The high molecular weight crystalline polymer produced with (P)-2-methylbutyllithium had a tritactic (di-iso-rra/w-tactic) structure. This was probably the first metal-catalyzed asymmetric polymerization 134). Polymerization of other dienes was attempted by using various asymmetric methods 135). 

Synthetic natural rubber, cw-polyisoprene, is an example of a stereospecific polymer made possible by this means. There are five types of stereo specific (or stereoregular) structures cis, trans, isotactic, syndi-otactic, and tritactic. 

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