Tail configuration

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 stmctures 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 stmctures also exists. Of these possible stmctures, only... 

The structure of the dimers from mero-substituted derivatives was initially determined by comparison of the observed and calculated dipole moments. For a head-to-tail dimer the dipole moments resulting from the 9 and 9 substituents should cancel each other and the resultant dipole moment should be essentially zero. For a head-to-head arrangement the dipoles would be in the same direction and the resultant should be considerably greater than zero. The dimers produced upon irradiation of 9-chloro and 9-bromoanthracene solutions were observed to be 0.36 and 0.60 D, respectively. Since these values are much less than expected for a head-to-head arrangement for these derivatives (3.8 D), it was concluded that both of these dimers were formed in a head-to-tail configuration/30 ... 

Propagation, or chain growth, takes place in a head-to-tail configuration as a result of resonance stabilization and steric factors by carbocation (M+) addition to another monomer molecule. The head stabilizes the cation best so it is the growing site while the least sterically... 

Figure 1.6. Portion of PP illustrating the head-to-tail configuration, which is typical for polymers.

Additional compression eventually leads to the collapse of the film. The pressure nc at which this occurs is somewhere in the vicinity of the equilibrium spreading pressure. Figure 7.7 represents schematically how this film collapse may occur. The mode of film buckling shown in Figure 7.7 is not the only possibility head-to-head as well as tail-to-tail configurations can be imagined. The second structure strongly resembles that of cell membranes, which we discuss in the next chapter. 

It is neither feasible nor appropriate in a book like this to give a detailed presentation of biological membranes, which compartmentalize living matter and perform numerous cell functions as well. However, because of the impetus to the study of surfactants that the membrane-mimetic properties of surfactant structures have provided, it would be a mistake to exclude some mention of membranes in this chapter. We have already noted in connection with Figure 7.7 that a monolayer may collapse into a bilayer that leaves the surfactant in a tail-to-tail configuration. This is exactly the arrangement of molecules in the lipid portion of a cell... 

Proposed organization of the enzymatic activities of fatty acid synthase from animal liver. Fatty acid synthase exists as a dimer of two giant identical peptides (Mr = 272,000). Each subunit has one copy of acyl carrier protein (ACP) and each of the enzyme activities involved in fatty acid synthesis is covalently linked. The two peptides are organized in a head-to-tail configuration in such a way that it is possible to make two fatty acid molecules at the same time. 

By a consecutive downstroke into the subphase through the floating monolayer a second layer can be transferred, with the alkyl chains oriented towards the solid substrate in a tail-to-tail configuration. 

Scheme 1.12 The preparation of 13, in which the ligands adopt a head-to-tail configuration.

The head-to-head and head-to-tail configuration of the isomeric ketones (7) and (8) were discernable. A slight distortion of the cyclobutane ring in compound (7) suggests an imbalance in steric strain <81CJC3136>. 

Figure 5.19. Example of a train-loop-tail configuration of an adsorbed chain (after Fleer et al., 1993).

When heated at 150 C in xylene for 24 h with an excess of phospholes, Santini showed that complexes of type XVI gave sandwich complexes of type XVIII with a head-to-tail configuration which were fully characterized including an X-ray crystal structure analysis. Depending on the starting materials, a number of mixed homo- and hetero-bimetallic sandwiches could be obtained. On the basis of structural and NMR data, it appears that there is a strong through-space interaction between the non-bonded iron and phosphorus atoms in these complexes. 

Poly (vinyl chloride) (PVC) is essentially a linear polymer with a head-to-tail configuration. Theoretical calculations and model compound studies predicted that PVC of ideal... 

Propagation. Propagation or chain growth takes place by successive addition of monomer molecules in a head-to-tail configuration. At low temperatures, the chain growth takes place rapidly, and the rate constant is essentially the same for all propagation steps. 

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