Chain sequence

In practice side reactions intervene to reduce the efficiency of the propagation steps The chain sequence is interrupted whenever two odd electron species combine to give an even electron product Reactions of this type are called chain terminating steps Some commonly observed chain terminating steps m the chlorination of methane are shown m the following equations... 

The final step of the oxidation process is the formation of CO2, and this also occurs by a number of chain sequences such as... 

Inclusion of the carbon atoms of an aromatic ring in the side chain sequence is apparently quite consistent with antlmalarial activity. Thus, reaction of p-acetamidophenol with formaldehyde and diethylamine affords the Mannich product, 79. This is then converted to the diamine (80) by saponification. Alkylation with the chloroquinoline, 70, affords amidoquine (81). The same sequence starting with the Mannich product in which pyrrolidine has been used as the amine (82) gives amopyroquine (83). 

In the schemes considered to this point, even the complex ones, the products form by a limited succession of steps. In these ordinary reaction sequences the overall process is completed when the products appear from the given quantity of reactants in accord with the stoichiometry of the net reaction. The only exception encountered to this point has been the ozone decomposition reaction presented in Chapter 5, which is a chain reaction. In this chapter we shall consider the special characteristics of elementary reactions that occur in a chain sequence. 

Wu, C., Sanborn, T., Huang, K., Zuckermann, R., and Barron, A. Peptoid oligomers with alpha-chiral, aromatic side chains Sequence requirements for the formation of stable peptoid helices. /. 

This comparison suggests that of these two similar reactions, only alkene additions are likely to be a part of an efficient radical chain sequence. Radical additions to carbon-carbon double bonds can be further enhanced by radical stabilizing groups. Addition to a carbonyl group, in contrast, is endothermic. In fact, the reverse fragmentation reaction is commonly observed (see Section 10.3.6) A comparison can also be made between abstraction of hydrogen from carbon as opposed to oxygen. 

Esters of AMiydroxypyridine-2-thione are another versatile source of radicals,286 where the radical is formed by decarboxylation of an adduct formed by attack at sulfur by the chain-carrying radical.287 The generalized chain sequence is as follows. 

The most general method for formation of new carbon-carbon bonds via radical intermediates involves addition of the radical to an alkene. The reaction generates a new radical that can propagate a chain sequence. The preferred alkenes for trapping alkyl... 

In this reaction, the trialkylstannane serves to initiate the chain sequence but it is present in low concentration to minimize the rate of hydrogen atom abstraction from the stannane. Under these conditions, the chain is propagated by iodine atom abstraction. 

A biologically formed protein or polynucleotide, while exhibiting some limited heterogeneity in length or microheterogeneity in the main chain sequence or in pendant prosthetic groups, has an essentially unique sequence. Different forms of a protein or polynucleotide often can be... 

Regiodefects are less readily incorporated into crystallites than defect-free chain sequences. In semicrystalline polymers, increasing levels of misinsertion result in reduced crystallinity. This can affect numerous physical properties, resulting in reduced modulus, lower heat distortion temperature, and decreased tensile strength. 

Copolymers are built from the repetition of two (or more) "repeat units". Depending on the spatial arrangement of those units (e.g., A and B) along the chain (sequencing), various types of copolymers can be made alternating, block, random (or statistical), branched, crosslinked (see Figure 12). 

More sophisticated microstructures are sometimes synthesized, but the highly complex chain sequencing encountered in natural copolymers, like proteins or... 

The low-frequency LAM in Raman spectroscopy is a vibrational mode pertaining to ordered sequences in the chain direction. The mode frequency (An) is inversely proportional to the ordered chain sequence length or to the crystallite thickness. This relation was given by Shimanouchi and co-workers [92,93] as ... 

This is done a contre coeur, since the L-chain sequence was the first to be established. However, it makes little sense to refer to the amino terminus of H-chain ferritins as —1, —2, etc. 

Crystallinity often has little if any effect on 7A, but with some polymers crystallized under certain conditions, the 7A value is raised (78,79). The increase appears to be caused either by polymer being restricted to short amorphous segments between two crystallites or by stresses put on the amorphous chain sequences as a result of the crystallization process. In either case the mobility is restricted, so higher temperatures are required to restore it. Thus quench cooling tends to increase 7A whereas annealing reduces TK back to the value typical of the amorphous polymer. 

Intermolecular hydride transfer (Reaction (6)), typically from isobutane to an alkyl-carbenium ion, transforms the ions into the corresponding alkanes and regenerates the t-butyl cation to continue the chain sequence in both liquid acids and zeolites. 

Fig. 3. Comparison of the amino acid sequences predicted from the a- and j3-tubulin mRNA sequences. Open circles indicate homologous, conservative substitutions, and boxed regions indicate areas of high homology. For completeness, the sequence of the first 25 amino acids of chick brain a-tubulin, deduced by the work of Luduena and Woodward (1973), is also included. Note that the asterisks indicate identical amino acids common to both polypeptide chain sequences. [Reproduced from Valenzuela et al. (1981). Nature (London) 289, 650-655.]...

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. 

Self-Assembly Assisted Polypolymerization (SAAP) A Novel Approach to Prepare Multiblock Copolymers with a Controllable Chain Sequence and Block... 

Figure 1. Schematic of self-assembly assisted polypolymerization (SAAP) of triblock copolymers in a selective solvent for the synthesis of long multiblock copolymers with a controllable chain sequence and block length. [3 5]...

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