Head-to-tail arrangement

At a fixed distance r, the angular factor in equation (A 1.5.12) leads to the greatest attraction when the dipoles are lined up in a linear head-to-tail arrangement, 9 whereas the Imear tail-to-tail geometry, 0 =... 

Generally polymers involve bonding of the most substituted carbon of one monomeric unit to the least substituted carbon atom of the adjacent unit in a head-to-tail arrangement. Substituents appear on alternate carbon atoms. Tacticity refers to the configuration of substituents relative to the backbone axis. In an isotactic arrangement, substituents are on the same plane of the backbone axis that is, the configuration at each chiral center is identical. 

The polymer is based on a simple head-to-tail arrangement of monomer units and is amorphous, since the specific position of the benzene ring is somewhat variable and hence inhibits crystallisation. Despite its generally desirable properties, for many applications it is considered too brittle. Because of this, a number of approaches have been made to modify the mechanical properties of poly (styrene). The most successful of these have been (i) copolymerisation and (ii) the addition of rubbery fillers. 

Figure 21-2. Fatty acid synthase multienzyme complex. The complex is a dimer of two identical polypeptide monomers, 1 and 2, each consisting of seven enzyme activities and the acyl carrier protein (ACP). (Cys— SH, cysteine thiol.) The— SH of the 4 -phosphopantetheine of one monomer is in close proximity to the— SH of the cysteine residue of the ketoacyl synthase of the other monomer, suggesting a "head-to-tail" arrangement of the two monomers. Though each monomer contains all the partial activities of the reaction sequence, the actual functional unit consists of one-half of one monomer interacting with the complementary half of the other. Thus, two acyl chains are produced simultaneously. The sequence of the enzymes in each monomer is based on Wakil.

Prevalence of the head-to-tail arrangement in vinyl polymers is abundantly confirmed by determinations of polymer structures. Staudinger and Steinhofer found that destructive distillation of polystyrene at about 300° yielded 1,3-diphenylpropane, 1,3,5-triphenyl-pentane, and 1,3,5-triphenylbenzene... 

The head-to-tail arrangement in poly-(vinyl alcohol) is further confirmed by its X-ray diffraction pattern in the crystalline state. Likewise, analysis of the X-ray diffraction of crystalline poly-(vinylidene chloride), (—CH2—CCI2—)x, and of crystalline (stretched) polyisobutylene, [—CH2—C(CH3)2—]x, shows the units to be arranged in these cases also in the expected head-to-tail forms. 

The crystal structure can be considered as a structure regularly stacked with bimolecular layers along the a-axis. Within the bimolecular layer, two molecules related by inversion symmetry face each other in the tail-to-tail fashion with their molecular axes inclined by about 26° to the bilayer surface. This inclination enables the head-to-tail arrangement of azobenzene chromophores as expected from the spectroscopic study. 

Before the mechanism of vinyl polymerization was understood, the question of the structure of vinyl polymers was of considerable interest. Staudinger had written these polymers as having a head-to-tail arrangement of recurring units, but he had not really furnished evidence of the structure. As Carothers once said, Staudinger had assigned the structure by pronouncement. He was as usual correct, and chemical evidence was developed to establish such structures. For example, when monovinyl methyl ketone polymerized, it could produce by head-to-head, tail-to-tail reaction a 1,4-diketone. By head-to-tail polymerization it would give a 1,5-diketone. These two types have different reactions. The study of the polymer proper showed that the polymer was a 1,5-diketone. In the case of polyvinyl chloride, a head-to-head, tail-to-tail polymerization would lead to a 1,2-dihalide compound, and a head-to-tail polymerization would lead to a 1,3-dihalide. 

Functional groups in the polymer backbone, such as the methyl group in PP, are called pendant groups. Such polymers are formed giving a head-to-tail arrangement rather than a head-to-head arrangement. 

The all-tra 5 -squalene (C30H50), discovered in shark liver oil in the 1920s, is a triterpene, but one in which the isoprene rule at violated in one point. Rather than a head-to-tail arrangement of six units of isoprene, there appear to be farnesyl units that have been connected tail to tail. Almost aU steroids are biosynthesized from cholesterol. Cholesterol is biosynthesized from squalene, which is first converted to lanosterol. The conversion of squalene to the steroid skeleton is an oxirane, squalene-2,3-oxide, which is transformed by enzymes into lanosterol, a steroid alcohol naturally found in wool fat. The whole process is highly stereoselective. 

Sesquiterpenes are formed by the head-to-tail arrangement of three isoprene units (15 carbon atoms) there are, however, many exceptions to the rule. Because of the complexity and diversity of the substances produced in nature, it is not surprising that there are many examples of skeletal rearrangements, migrations of methyl groups, and even loss of carbon atoms to produce norsesquiterpenoids. 

Bidentate N C donor ligands display head to tail arrangement in homoleptic dinuclear complexes [Au2(N C)2] that include [ 2 ( 2 ) 2]2+ [CH3im... 

A pertinent question is whether the structure of base-initiated polymer corresponds to the idealized head-to-tail arrangement that seems characteristic of radical-initiated polymer. General features of the infrared spectrum are identical for the two types. However, unreported work in this laboratory (119) suggests that structures of the type... 

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