Regular and irregular chains

The second requirement cannot be met unless the first is met, but chemical regularity does not guarantee that the second requirement can be met. 

Chemical regularity means that the chain must be made up of identical chemical repeat units, i.e. units with the same chemical and structural for- 

Stereoregularity means that the chain must be capable (by rotations about single bonds) of taking up a structure with translational symmetry. Translational symmetry means that the chain is straight on a scale large compared with the size of the chemical repeat unit and consists of a regular series of translational repeat units, so that the chain can be superimposed upon itself by translation parallel to the axis of the chain by the length of one translational repeat unit (see fig. 4.1). The translational repeat unit may be identical to the chemical repeat unit, but often there is more than one chemical repeat unit in a translational repeat unit. The most important 

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The phenomenon is found with many, but not all substituted polythiophenes. Studies of 3,3 -di-alkyl-substituted polythiophenes revealed no temperature-induced colour change [17]. But many other structures will give thermochromism and solvatochromism. Polythiophenes substituted both with alkoxy and alkyl side chains may show it both purely amorphous and partially crystalline polythiophenes show it short and long-chain polymers may show it both regular and irregular polymers show it (see Figure 15.1),... 

The mesomorphous phase, also called an intermediate phase or a mesophase, is formed by molecules occurring in surface layers of the crystallites. It can be assumed that the mesophase is made up largely by regularly adjacent reentry folds. However, it cannot be excluded that the mesophase is also composed of some irregular chain folds, which are characterized by a long length and run near the crystal face in the direction perpendicular to the microfibril axis. 

Indeed, in the world of tomorrow we can expect new aspects of polymer solids to extend the conventional and successful structure ideas of this century. These, of course, were the recognition as molecular identities of the chains of repeating chemical monomers. The circumstances of those entities have resulted in interesting concepts of solubilities, viscosity, and other mechanics, and especially thermodynamic limitations m mutual solubility or comparability of polymer mixtures. But we have known for decades that even homogeneous regular chain polymers such as Carothers polyesters and polyamides formed solids with manifold imperfections and irregularities, such as order-disorder crystal configurations.(22,23)... 

Block copolymers are named by using dashes (double-length hyphens) for the bonding of bloeks with each other and with junction units. With graft and star polymers, the grafts or the arms, respectively, are eonsidered to be substituents to the main chain, and the structure is named in the same way as a regular or irregular polymer. Table 8 lists some examples. 

Inserted L-rhamnopyranosyl units may provide the necessary irregularities (kinks) in the structure required to limit the size of the junction zones and produce a gel. The presence of side chains composed of D-xylosyl units may also be a factor that limits the extent of chain association. Junction zones are formed between regular, unbranched pectin chains when the negative charges on the carboxylate groups are removed (addition of acid), hydration of the molecules is reduced (addition of a cosolute to a solution of HM pectin), and/or pectinic acid polymer chains are bridged by multivalent, eg, calcium, cations. 

Fig. 24a and b. Schematic simplified representation of the basic difference between a regular a and irregular b network knots (1), chains connecting nearest-neighbor knots (2), pendant chains (3)... 

Polymeric compounds with limited Si-Si units in which the polymeric character is due to other bond systems, have been described. Another series of polymeric compounds exists, in which the Si-Si bonds themselves are responsible for the polymeric state. The extreme case, when all four valencies of a silicon atom are bonded with Si atoms, produces metallic silicon with a diamond structure. Compounds of the type (SiX2) or (SiX) result when some valencies of each silicon atom are occupied by other atoms or groups. Only polymeric compounds (SiXj,) withy from 2 to 1 exist SiX3 yields disilanes, and compounds withy between 3 and 2 yield limited chains. The possible stoichiometric compositions SiX2 and SiX are sometimes found, but a non-stoichio-metric composition is more common, which is understandable in view of the irregular framework of the Si-Si structure. The formation of regular or irregular Si-Si structures of stoichiometric or non-stoichiometric composition will depend on the procedures used in preparation. 

The regular secondary structures, a helices and /i sheets, are connected by coil or loop regions of various lengths and irregular shapes. A variant of the loop is the f> turn or reverse turn, where the polypeptide chain makes a sharp, hairpin bend, producing an antiparallel / turn in the process. 

The complete pattern of folding of the polypeptide chain of a protein, whether regular or irregular, is called the tertiary structure. The tertiary structure of any protein is the sum of many forces and structural elements, many of which are the result of interactions between the side chain groups of amino acids in the protein. Some of these interactions are described below. 

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